qtpy: use our internal parse() as a fallback when packaging.version.parse() is not installed

Continue relying on packaging.version.parse() when it is installed.
Our internal parsing code is now used as a fallback instead.

Suggested-by: @dalthviz in #508

Author: davvid

qtpy/__init__.py

@@ -194,7 +194,12 @@ def _parse_int(value):
 
 def _parse_version(version):
     """Parse a version string into a tuple of ints"""
-    return tuple(_parse_int(x) for x in version.split("."))
+    try:
+        from packaging.version import parse as _packaging_version_parse
+    except ImportError:
+        return tuple(_parse_int(x) for x in version.split("."))
+    else:
+        return _packaging_version_parse(version)
 
 
 # Unless `FORCE_QT_API` is set, use previously imported Qt Python bindings