Fix parsing of package versions in packaging 22. (#181)

Co-Authored-By: Simon Cross <165551+hodgestar@users.noreply.github.com>

setup.py

@@ -42,7 +42,10 @@ def _determine_version(options):
     with open(version_filename, "r") as version_file:
         version_string = version_file.read().strip()
     version = packaging.version.parse(version_string)
-    if isinstance(version, packaging.version.LegacyVersion):
+    # 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)
     options['short_version'] = str(version.public)
     options['release'] = not version.is_devrelease