Prepare 2.4.1 release with an installation failure fix

This addresses the following error happening during installation
of the package:

    Collecting pygtrie
      Downloading pygtrie-2.4.0.tar.gz (35 kB)
        ERROR: Command errored out with exit status 1:
         command: /usr/local/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-p_3v3rdq/pygtrie/setup.py'"'"'; __file__='"'"'/tmp/pip-install-p_3v3rdq/pygtrie/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-yevdgn7f
             cwd: /tmp/pip-install-p_3v3rdq/pygtrie/
        Complete output (5 lines):
        Traceback (most recent call last):
          File "<string>", line 1, in <module>
          File "/tmp/pip-install-p_3v3rdq/pygtrie/setup.py", line 5, in <module>
            import packaging.version
        ModuleNotFoundError: No module named 'packaging'
        ----------------------------------------
    ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Fixes: google#32

setup.py

@@ -2,7 +2,6 @@
 import distutils.command.build_py
 import distutils.command.sdist
 import distutils.core
-import packaging.version
 import os
 import os.path
 import re
@@ -128,12 +127,12 @@ def get_readme_lines():
     yield '\n'
 
     with codecs.open('version-history.rst', 'r', 'utf-8') as fd:
-        min_version = packaging.version.parse('2.0')
-        version_re = re.compile(r'^- ([0-9.]*):')
+        min_major_version = 2
+        version_re = re.compile(r'^([0-9]+)\.[0-9.]+:')
         cleanup_re = re.compile(r':(?:class|func|const):`([^`]*)`')
         for line in fd:
             m = version_re.search(line)
-            if m and packaging.version.parse(m.group(1)) < min_version:
+            if m and int(m.group(1)) < min_major_version:
                 break
             line, _ = cleanup_re.subn(r'``\1``', line)
             yield line
@@ -146,7 +145,6 @@ def get_readme_lines():
     'name': 'pygtrie',
     'version': release,
     'description': 'A pure Python trie data structure implementation.',
-    'long_description_content_type': 'text/x-rst',
     'long_description': readme,
     'author': 'Michal Nazarewicz',
     'author_email': '[email protected]',

version-history.rst

@@ -1,7 +1,13 @@
 Version History
 ---------------
 
-2.4.0: 2020/11/19
+2.4.1: 2020/11/20
+
+- Remove dependency on ``packaging`` module from ``setup.py`` to fix
+  installation on systems without that package.  [Thanks to Eric
+  McLachlan for reporting]
+
+2.4.0: 2020/11/19  [pulled back from PyPi]
 
 - Change ``children`` argument of the ``node_factory`` passed to
   :func:`Trie.traverse` from a generator to an iterator with a custom