Simplify version parsing.

Calling Version directly has given the strict parsing we require
since the Version class was created and the class predates the
parse(...) function itself. This simpler code should give the
correct behaviour on all versions of packaging since 2014.

setup.py

@@ -132,12 +132,7 @@ def _determine_version(options):
     version_filename = os.path.join(options['rootdir'], 'VERSION')
     with open(version_filename, "r") as version_file:
         version_string = version_file.read().strip()
-    version = packaging.version.parse(version_string)
-    # LegacyVersion was removed in packaging 22, but is still returned by
-    # packing <= 21
-    LegacyVersion = getattr(packaging.version, "LegacyVersion", type(None))
-    if isinstance(version, LegacyVersion):
-        raise ValueError("invalid version: " + version_string)
+    version = packaging.version.Version(version_string)
     options['short_version'] = str(version.public)
     options['release'] = not version.is_devrelease
     if not options['release']: