Python 3.11 support, drop 3.6 and older

major

• Support Python 3.9, 3.10, 3.11. Drop Python 3.6 and earlier, All code
  and examples have been updated to the new-style PEP-492 syntax.
• the loop, event_loop, and log arguments are no longer accepted to
  any class initializers.
• The asyncio Future, TelnetServer.waiter_encoding, now receives
  only a boolean, rather than an instance of TelnetServer (self).
• This release has a known memory leak when using the _waiter_connected and
  _waiter_closed arguments to Client or Shell class initializers, please do
  not use them. A replacement "wait_for_negotiation" awaitable will be provided
  in a future release.

minor

• std. python telnetlib is no longer imported, its constants are
  extracted directly to avoid DeprecationWarning
• some changes in CI/tox about dropping/adding python support, etc.
• skip pexpect tests with py3.10 until I can find the time to also
  devote time to maintain that library to fix asyncio compatibility
  issues
• small fix in repr_mapping() to use shlex to make values containing
  spaces conditionally quoted
• small race condition fix for waiter_closed
• small race condition fix for repr(BaseClient) and Server,
  around "peername" and in get_extra_info()