bpo-30096: Use ABC in abc reference examples by appeltel · Pull Request #1220 · python/cpython

Update example code to use abc.ABC rather than abc.ABCMeta
to define abstract base classes. Add examples using both
methods in the documentation for abc.ABC. Reorder the
documentation of the classes so that the examples make sense
when the document is read from top to bottom.

* 'master' of https://github.com/python/cpython: (601 commits)
  remove check for bug last seem in Solaris 9 (python#3285)
  Change code owners for hashlib and ssl to the crypto team (python#3284)
  bpo-31281: Fix pathlib.Path incompatibility in fileinput (pythongh-3208)
  remove autoconf check for select() (python#3283)
  remove configure check for 'volatile' (python#3281)
  Add missing _sha3 module to Setup.dist (python#2395)
  bpo-12383: Also ignore __PYVENV_LAUNCHER__ (python#3278)
  bpo-9146: add the missing NEWS entry. (python#3275)
  Fix a c.f.as_completed() refleak previously introduced in bpo-27144 (python#3270)
  bpo-31185: Fixed miscellaneous errors in asyncio speedup module. (python#3076)
  remove a redundant lower in urllib.parse.urlsplit (python#3008)
  bpo-31323: Fix reference leak in test_ssl (python#3263)
  bpo-31250, test_asyncio: fix EventLoopTestsMixin.tearDown() (python#3264)
  bpo-31326: ProcessPoolExecutor waits for the call queue thread (python#3265)
  bpo-27144: concurrent.futures as_complete and map iterators do not keep reference to returned object (python#1560)
  bpo-31250, test_asyncio: fix dangling threads (python#3252)
  bpo-31217: Fix regrtest -R for small integer (python#3260)
  bpo-30096: Use ABC in abc reference examples (python#1220)
  bpo-30737: Update DevGuide links to new URL (pythonGH-3228)
  [Trivial] Remove now redundant assert (python#3245)
  ...