Parse alpha, beta and rc versions strings.

fixes #142

test/test_utils.py

@@ -89,6 +89,9 @@ def test_bytes_to_unicode_failing_encoding(code, errors):
         ('3.6', (3, 6)),
         ('3.6.10', (3, 6)),
         ('3.10', (3, 10)),
+        ('3.10a9', (3, 10)),
+        ('3.10b9', (3, 10)),
+        ('3.10rc9', (3, 10)),
     ]
 )
 def test_parse_version_string(version_str, version):