Fix bad ref count dec in Python APIs font index/contains

Python.c had a few places where PYGETSTR()/ENDPYGETSTR() defines were
not used correctly, and worse, a couple places where Py_DECREF() was
used on the wrong object rather than correctly using ENDPYGETSTR().

When building FontForge with Python3 and running Python scripts, this
could result in active variables being garbage-collected out from
under running code.  Oh, and leaking memory.  This only happened with
Python3, not Python2.

Test failures and FF crashes could be demonstrated when using the construct
    glyph = font1[glyphName]
as the problems in PyFF_FontIndex() were a bit worse.

Failures couldn't be demonstrated as easily when using the construct
    if glyphName in font1:
though the code in PyFF_FontContains() was similarly wrong.

It would seem as though this code was done before the PYGETSTR/ENDPYGETSTR
ideas were fully worked out?  As part of enabling the code for Python3
compatibility.  Certainly fully using those defines makes the code shorter.

This change has been tested under Python 2.7.6 and Python 3.4.0 (Ubuntu):
* with Python2 w/o fix - success
* with Python3 w/o fix - gross failures and "Segmentation fault (core dumped)"
* with Python3 w/fix   - success
* with Python2 w/fix   - success

A test script is also added, test1008.py, that tests a number of interrelated
APIs for scanning all glyphs in a font:

  # Checking these API operations from python.c:
  #
  #   if integer in f:          - font has a glyph for this encoding value?
  #   glyph = f[integer]        - get glyph object for encoding value index
  #   for name in f:            - iterate through glyph names in font
  #   glyph = f[name]           - get glyph object for glyph name key
  #   for glyph in f.glyphs()
  #                             - iterate thru glyphs in GID order
  #   for glyph in f.glyphs('encoding')
  #                             - iterate thru glyphs in encoding order
  #   if name in f:             - font has a glyph for this glyph name?

I haven't fully checked out all usages of PYGETSTR/ENDPYGETSTR, and routines
GlyphsFromTuple() and maybe PyFF_PrivateIndexAssign() look needy.  Nor have
I checked on all uses of Py_DECREF(), just enough to say not misused a lot.
Merge pull request #1548 from tshinnic/font_index_contains_decref_fix

Fix bad ref count dec in Python APIs font index/contains