Install cysignals-CSI-helper as package data for better portability

Rather than installing `cysignals-CSI-helper.py` into a `share`
directory and then trying to figure out the correct path to it, install
it as Python package data and use the standard `importlib.resources`
API to access it.  For Python versions older than 3.9,
the `importlib_resources` backport is used instead.

Fixes sagemath#200

setup.py

@@ -215,12 +215,14 @@ def run(self):
     packages=["cysignals"],
     package_dir={"": "src"},
     package_data={"cysignals": ["*.pxd", "*.h"]},
-    data_files=[(opj("share", "cysignals"), [opj("src", "scripts", "cysignals-CSI-helper.py")])],
     scripts=glob(opj("src", "scripts", "cysignals-CSI")),
     cmdclass=dict(
         configure=configure,
         build_py=build_py,
         build_ext=build_ext,
         bdist_egg=no_egg
     ),
+    install_requires=[
+        "importlib_resources; python_version < '3.9'",
+    ],
 )

src/scripts/cysignals-CSI

@@ -41,6 +41,11 @@ from argparse import ArgumentParser
 from datetime import datetime
 from distutils.spawn import find_executable
 
+if sys.version_info >= (3, 9):
+    import importlib.resources as importlib_resources
+else:
+    import importlib_resources
+
 
 def pid_exists(pid):
     """
@@ -65,15 +70,12 @@ def gdb_commands(pid, color):
     cmds += b'import sys; sys.stdout.flush()\n'
     cmds += b'end\n'
     cmds += b'bt full\n'
-    cysignals_share = os.path.join(os.path.dirname(sys.argv[0]), '..',
-                                   'share', 'cysignals')
-    script = os.path.join(cysignals_share, 'cysignals-CSI-helper.py')
-    with open(script, 'rb') as f:
-        cmds += b'python\n'
-        cmds += b'color = %r; ' % color
-        cmds += b'sys_path = %r; ' % sys.path
-        cmds += f.read()
-        cmds += b'end\n'
+    script = importlib_resources.files('cysignals') / 'cysignals-CSI-helper.py'
+    cmds += b'python\n'
+    cmds += b'color = %r; ' % color
+    cmds += b'sys_path = %r; ' % sys.path
+    cmds += script.read_bytes()
+    cmds += b'end\n'
     cmds += b'detach inferior 1\n'
     cmds += b'quit\n'
     return cmds