lang/pypy: add separate ports for pypy-gdbm, pypy-sqlite3 and pypy-tkinter

[dbn]

Summary

Add the gdbm port (a la python) for PyPy. Since it is unprecedented having a PyPy dependent port (but with lots of precedents regarding python dependent ports) I thought it would be best to give this a thorough review.

Other than the usual I would like to request review on:

• The 'ppy' suffix: If/when the python infrastructure grows support for more implementations (such as pypy, pyston, stackless, ironpython and jpython) there could possibly be a need for some implementation specific ports (such as numpy, gdbm, etc). These specific ports (and ports build against the alternative implementations) could be differentiated by having a suffix for the suffix (such as p, s?, s?, i, j respectively).
• The '26' suffix: now that I think about it I think this version of the PyPy implementation should be dropped as it would conflict with PyPy3 (i.e. I suggest ppy- and ppy3- suffix for lang/pypy and lang/pypy3 respectively).
• The need for a separate port: This port could possibly be merged into the py-gdbm port (possibly with different Makefile.py or Makefile.ppy ports)? I think it is cleaner to have different ports altogether
• Integration into Mk/Uses: Any thoughts into how one would go about adding support for this in the Ports Framework?

Test Plan

• Portlint [done]
• Port test [done]
• Poudriere

[koobs]

My thoughts on this are:

• Prefer pypy- as the prefix
  • Would act similarly to pyXY- prefix for python port packages build for Python X.y version
• If pypy 'versions' are materially same as the same underlying package, then adding support for producing pypy-* versions of said port feels like a good place to start
• Any pypy- specific framework routines should use/keep as many Python conventions/bits as possible/practical