Prefer public packaging module to pkg_resources' private copy of it (P…

…ython-Markdown#826)

Some distributions of `pkg_resources` remove the vendored copy of `packaging. Additionally, the vendored copy is not publicly documented and should not be relied on exclusively.  Regardless, if `pkg_resources` is installed at all, then we can be fairly certain one of `packaging` or `pkg_resources._vendor.packaging` will be available. That being the case, there is no need to list `packaging` as a dependency.

Fixes Python-Markdown#825.

docs/change_log/index.md

@@ -7,6 +7,8 @@ _______, 2019: Released version 3.1.1 (a bug-fix release).
 
 * Fixed import failure in `setup.py` when the source directory is not
   on `sys.path` (#823).
+* Prefer public `packaging` module to pkg_resources' private copy of
+  it (#825).
 
 Mar 25, 2019: Released version 3.1 ([Notes](release-3.1.md)).
 

markdown/__meta__.py

@@ -20,7 +20,10 @@
 License: BSD (see LICENSE.md for details).
 """
 
-from pkg_resources.extern import packaging
+try:
+    import packaging.version
+except ImportError:
+    from pkg_resources.extern import packaging
 
 # __version_info__ format:
 # (major, minor, patch, dev/alpha/beta/rc/final, #)