Disallow creating 2.1 pattern with StringConstants in place of TimestampConstants

[clenk]

When manually creating pattern objects using the classes in https://github.com/oasis-open/cti-python-stix2/blob/master/stix2/patterns.py, it is possible to create invalid STIX 2.1 patterns that use a StringConstant where they should use a TimestampConstant.

This doesn't affect parsing of patterns, and the pattern validator should still catch this error. Thus, this is deemed to be a low priority issue currently, but feel free to leave a comment if this is something you need fixed.

Related: #400