sagemathgh-38914: Replace os.uname by more universal platform.system

    
`os.uname` is not available on Windows, so we replace it by
`platform.system`. Also replace the `UNAME` variable in `sage.env` which
is was only used in one place.

Extracted from sagemath#38872

<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->



### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#38914
Reported by: Tobias Diez
Reviewer(s): Kwankyu Lee

src/sage/doctest/external.py

@@ -32,12 +32,12 @@
 #*****************************************************************************
 
 import multiprocessing
-import os
+import platform
 
 # With OS X, Python 3.8 defaults to use 'spawn' instead of 'fork' in
 # multiprocessing, and Sage doctesting doesn't work with 'spawn'. See
 # trac #27754.
-if os.uname().sysname == 'Darwin':
+if platform.system() == 'Darwin':
     multiprocessing.set_start_method('fork', force=True)
 Array = multiprocessing.Array
 

src/sage/doctest/forker.py

@@ -48,6 +48,7 @@
 
 
 import os
+import platform
 import sys
 import time
 import signal
@@ -80,7 +81,7 @@
 # With OS X, Python 3.8 defaults to use 'spawn' instead of 'fork' in
 # multiprocessing, and Sage doctesting doesn't work with 'spawn'. See
 # trac #27754.
-if os.uname().sysname == 'Darwin':
+if platform.system() == 'Darwin':
     multiprocessing.set_start_method('fork', force=True)
 
 

src/sage/env.py

@@ -36,6 +36,7 @@
 
 from typing import Optional
 import sage
+import platform
 import os
 import socket
 import sys
@@ -165,7 +166,6 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 
 
 # system info
-UNAME = var("UNAME", os.uname()[0])
 HOSTNAME = var("HOSTNAME", socket.gethostname())
 LOCAL_IDENTIFIER = var("LOCAL_IDENTIFIER", "{}.{}".format(HOSTNAME, os.getpid()))
 

src/sage/interfaces/singular.py

@@ -335,6 +335,7 @@
 # ****************************************************************************
 
 import os
+import platform
 import re
 import sys
 import pexpect
@@ -407,7 +408,7 @@ def __init__(self, maxread=None, script_subdirectory=None,
                         restart_on_ctrlc=True,
                         verbose_start=False,
                         logfile=logfile,
-                        eval_using_file_cutoff=100 if os.uname()[0] == "SunOS" else 1000)
+                        eval_using_file_cutoff=100 if platform.system() == "SunOS" else 1000)
         self.__libs = []
         self._prompt_wait = prompt
         self.__to_clear = []   # list of variable names that need to be cleared.

src/sage/misc/viewer.py

@@ -26,6 +26,7 @@
 ---------------------
 """
 
+import platform
 from sage.structure.sage_object import SageObject
 
 
@@ -66,7 +67,7 @@ def default_viewer(viewer=None):
         PDF_VIEWER = BROWSER
         PNG_VIEWER = BROWSER
 
-    elif os.uname()[0] == 'Darwin':
+    elif platform.system() == 'Darwin':
         # Simple on OS X, since there is an open command that opens
         # anything, using the user's preferences.
         BROWSER = 'open -W'

src/sage_docbuild/utils.py

@@ -4,6 +4,7 @@
 
 import errno
 import os
+import platform
 import traceback
 from typing import Optional
 
@@ -193,7 +194,7 @@ def build_many(target, args, processes=None):
     # With OS X, Python 3.8 defaults to use 'spawn' instead of 'fork'
     # in multiprocessing, and Sage docbuilding doesn't work with
     # 'spawn'. See trac #27754.
-    if os.uname().sysname == "Darwin":
+    if platform.system() == "Darwin":
         set_start_method("fork", force=True)
     from queue import Empty
 

src/sage_setup/setenv.py

@@ -1,7 +1,7 @@
 # Set some environment variables in the running process
 
 import os
-import sage.env
+import platform
 from pathlib import Path
 
 def _environ_prepend(var, value, separator=':'):
@@ -12,7 +12,7 @@ def _environ_prepend(var, value, separator=':'):
             os.environ[var] = value
 
 def setenv():
-    from sage.env import UNAME, SAGE_LOCAL, SAGE_VENV, SAGE_ARCHFLAGS, SAGE_PKG_CONFIG_PATH
+    from sage.env import SAGE_LOCAL, SAGE_VENV, SAGE_ARCHFLAGS, SAGE_PKG_CONFIG_PATH
 
     ##
     ## from sage-env:
@@ -32,7 +32,7 @@ def setenv():
         _environ_prepend('CPATH',        f'{SAGE_LOCAL}/include')
         _environ_prepend('LDFLAGS',      f'-L{SAGE_LOCAL}/lib -Wl,-rpath,{SAGE_LOCAL}/lib',
                          separator=' ')
-        if UNAME == 'Linux':
+        if platform.system() == 'Linux':
             _environ_prepend('LDFLAGS',      f'-Wl,-rpath-link,{SAGE_LOCAL}/lib',
                              separator=' ')
         if Path(SAGE_VENV).resolve() != Path(SAGE_LOCAL).resolve():