python-setuptools: upgrade to 38.2.4; use pypi.bbclass; improvements

python-setuptools: upgrade to 38.2.4; use pypi.bbclass; improvements

* Simplify python- and python3-setuptools with pypi.bbclass
* inherit setuptools rather than distutils
* Consolidate common settings in python-setuptools.inc
  - use PYTHON_PN variable to eliminate duplication
  - python3-setuptools had missing RDEPENDS (e.g., plistlib)
  - installer no longer creates setuptools.pth, drop fixes

(From OE-Core rev: 624a6f209248a0c8e0759d43f246c903a8db6a71)

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>