Parse alpha, beta and rc versions strings.

fixes #142

parso/utils.py

@@ -129,7 +129,7 @@ def version_info():
 
 
 def _parse_version(version):
-    match = re.match(r'(\d+)(?:\.(\d{1,2})(?:\.\d+)?)?$', version)
+    match = re.match(r'(\d+)(?:\.(\d{1,2})(?:\.\d+)?)?((a|b|rc)\d)?$', version)
     if match is None:
         raise ValueError('The given version is not in the right format. '
                          'Use something like "3.8" or "3".')

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):