Merge pull request #2039 from hodgestar/fix/v5-fix-legacy-version-check

Fix parsing of version number when using packaging 22.

setup.py

@@ -142,8 +142,11 @@ 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):
-        raise ValueError("invalid version: " + 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)    
     options['short_version'] = str(version.public)
     options['release'] = not version.is_devrelease
     if not options['release']: