fix: gh#172 use default date values

When parsing the current version, if it doesn't
specify anyt date part, (such as is the case for
e.g. SemVer), then use the current date to
populate default parts.

This enables updating YYYY patterns in copyright
headers even for projects that don't use a CalVer
pattern.

test/test_cli.py

@@ -1200,7 +1200,7 @@ def test_multimatch_file_patterns(config_text, runner):
 
 
 def _kwargs(year, month, minor=False):
-    return {'date': dt.date(year, month, 1), 'minor': minor}
+    return {'maybe_date': dt.date(year, month, 1), 'minor': minor}
 
 
 ROLLOVER_TEST_CASES = [