Allow 3-part version string lookups, e.g 2.7.1

We allow a float here, but if passed a string like
'2.7'. or  '2.7.13', accept that in looking up
either a scanner or a parser.

pytest/test_basic.py

@@ -0,0 +1,11 @@
+from uncompyle6.scanner import get_scanner
+from uncompyle6.parser import get_python_parser
+
+def test_get_scanner():
+    # See that we can retrieve a scanner using a full version number
+    assert get_scanner('2.7.13')
+
+
+def test_get_parser():
+    # See that we can retrieve a sparser using a full version number
+    assert get_python_parser('2.7.13')