Deprecate FreeType 2.7

[hugovk]

Fixes #5075.

Changes proposed in this pull request:

• Add deprecation warning when using ImageFont with FreeType older than 2.8

• Autolink CVEs with https://github.com/sloria/sphinx-issues

  • :cve:CVE-2020-15999 becomes https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-15999

• Autolink a PEP with https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#role-pep

  • :pep:527 becomes https://www.python.org/dev/peps/pep-0527

• Update Makefile docs command for Python 3

[radarhere]

@hugovk so #5101 has been merged, but it doesn't show up in the master commits, which is weird. The explanation though is in this PR, where it says that you force pushed master? I presume this is just an accident?

[hugovk]

Arg, that was my fault. I meant to update master in my security repo, and with less CI jobs enabled, but instead it went to upstream. So I force pushed to undo the last commit here and wiped out #5101 by mistake. Sorry about that! I'll redo #5101 later unless you get to it first.

[eli-schwartz]

After this PR:

$ python
Python 3.9.4 (default, Apr 20 2021, 15:51:38) 
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import PIL.ImageFont
>>> PIL.ImageFont.FreeTypeFont()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/site-packages/PIL/ImageFont.py", line 173, in __init__
    freetype_version = parse_version(features.version_module("freetype2"))
  File "/usr/lib/python3.9/site-packages/packaging/version.py", line 57, in parse
    return Version(version)
  File "/usr/lib/python3.9/site-packages/packaging/version.py", line 296, in __init__
    match = self._regex.search(version)
TypeError: expected string or bytes-like object

The ImageFont module tries to be accepting of missing (optional) system dependencies for the _imagingft module. But then features.version_module() returns None, which cannot be parsed as a version, so this dumps a completely incomprehensible traceback rather than what most of the rest of this module does, which is:

Pillow/src/PIL/ImageFont.py

Lines 41 to 50 in 5673090

	class _imagingft_not_installed:
	# module placeholder
	def __getattr__(self, id):
	raise ImportError("The _imagingft C module is not installed")
	try:
	from . import _imagingft as core
	except ImportError:
	core = _imagingft_not_installed()

[radarhere]

This bug has now been fixed by #5445