Add version specified ports of separated standard Python modules for
non-default Python versions:

• Add pyXY-{sqlite3,gdbm,tkinter} ports for generating binary packages
• Improve/add pkg-message to point users to install respective packages of separated Python standard modules
• Add COMMENT to explicitly show the Python version that package should be used with
• Simplify version-related PYTHON_* for lang/python35

Reviewed by: koobs
Differential Revision: https://reviews.freebsd.org/D4170