databases/py-bsddb3: Convert to master port for databases/py3-bsddb3

• Convert databases/py3-bsddb3 from stand-alone port to slave of databases/py-bsddb3
• Prevent databases/py-bsddb3 from clobbering USES from databases/py3-bsddb3
• Convert USE_BDB=yes to USES=bdb in both databases/py-bsddb3 and databases/py3-bsddb3

PR: 208285
Reviewed by: mat (mentor)
Approved by: bofh (maintainer), adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D6614