Remove some more expired ports:

devel/py-backports.functools_lru_cache
devel/py-backports_abc
devel/py-futures
devel/py-singledispatch
devel/py-typing
graphics/py-pillow6

Adjust dependent ports.
Tested with 'make index' against r559975

Revert r559792 to unbreak INDEX and bulk -a
It seems a lot of reverse dependencies were missed

With hat:	portmgr

Remove most expired ports that depend on Python 2.7:

Exceptions are:
- mail/mailman, our listservers still use it
- www/chromium, solely because of popular demand,
  upstream is (slowly) working on porting it to Python 3
- ports that have actual patches for Python 3 against
  them (Cinnamon and a few unrelated other ports)
- llvm70 and qt5-webengine because they seem to have a lot
  of dependencies which need further analysis
- sysutils/uefi-edk2-bhyve because bhyve seems to use them
- python2 and python27 themselves

I'm sure there will be complaints, but the nice thing about
a version control system is that we can revert and adjust things.

Limit py-pillow6 to python 2.7 to eliminate possible conflict with py-pillow

With hat:	portmgr

graphics/py-pillow: Update to 7.0.0

* Repo-Copy the Pillow 6.2.2 release to graphics/py-pillow6 to retain
  backwards compatibility for Python 2 consumers as the Pillow 7.0.0 release
  dropped the support for Python 2.

* Apply conditional statements to use either Pillow 7.x or Pillow 6.x for
  consumers that can be built for Python 2 or newer.

* Exceptions are ports that can be built only for either Python 2 or
  Python 3.  For the first case, consumers are just assigned to the
  repo-copied graphics/py-pillow6.

* Also remove Pillow from BUILD_DEPENDS of math/py-PyWavelets as it is not
  listed in setup.py as a build dependency [1] and relax the version