Move ngettext alias to a shared module

To be used by other modules.

augur/io/print.py

@@ -1,3 +1,4 @@
+import gettext
 import sys
 
 from augur.debug import DEBUGGING
@@ -13,3 +14,9 @@ def print_debug(*args):
     """Print to stderr if in debugging mode."""
     if DEBUGGING:
         print_err(*args)
+
+
+# Use ngettext() without a message catalog for its singular/plural handling so
+# we can make proper error messages.  gettext() (no "n") is conventionally
+# aliased as "_", so alias ngettext() as "_n".
+_n = gettext.NullTranslations().ngettext

augur/merge.py

@@ -32,7 +32,6 @@
 you want to use a version different from what's on PATH), set the SQLITE3
 environment variable to path of the desired sqlite3 executable.
 """
-import gettext
 import os
 import re
 import subprocess
@@ -48,19 +47,13 @@
 from augur.argparse_ import ExtendOverwriteDefault, SKIP_AUTO_DEFAULT_IN_HELP
 from augur.errors import AugurError
 from augur.io.metadata import DEFAULT_DELIMITERS, DEFAULT_ID_COLUMNS, Metadata
-from augur.io.print import print_err, print_debug
+from augur.io.print import print_err, print_debug, _n
 from augur.utils import first_line
 
 
 T = TypeVar('T')
 
 
-# Use ngettext() without a message catalog for its singular/plural handling so
-# we can make proper error messages.  gettext() (no "n") is conventionally
-# aliased as "_", so alias ngettext() as "_n".
-_n = gettext.NullTranslations().ngettext
-
-
 class NamedMetadata(Metadata):
     name: str
     """User-provided descriptive name for this metadata file."""