Commit adfb650 removed support for aiohttp 2.x and Python
3.5 but there are still some remnants of these left. We get rid of them.
This also removes one `mypy` warning:

> compat.py:31: error: All conditional function variants must have identical signatures [misc]

We no longer run tests against aiohttp 2.x so we don't need it in
`tox.ini`.

aioresponses works with Python 3.10 with no issues so we update the
package metadata to say that, and we update the list of test
environments to include Python 3.10.

aioresponses works with aiohttp 3.8 so we extend the list of test
environments to include this version.

We cannot run tests with Python 3.10 using Pytest 6.x due to [1]. This
is fixed in Pytest 7.x but Pytest 7.x no longer supports Python 3.6
which aioresponses tries to support. However because Python 3.6 is
already past the EOL date, the simplest solution is to no longer test
with Python 3.6 and to stop declaring support for that version.

[1] pytest-dev/pytest#8546

Our tests are filing on Python 3.10 due to [1] so we update Pytest in
`requirements-dev.txt` to the latest version.

[1] pytest-dev/pytest#8546

aiohttp 3.6 and older use Python features which were removed in Python
3.10:

                assert port is not None
    >           hosts = await asyncio.shield(self._resolve_host(
                    host,
                    port,
                    traces=traces), loop=self._loop)
    E               TypeError: shield() got an unexpected keyword argument 'loop'

    .tox/py3.10-aiohttp35/lib/python3.10/site-packages/aiohttp/connector.py:952: TypeError

To avoid test failures, we remove unsupported combinations from GitHub
workflows.

aiohttp 3.2.x and older don't work on Python 3.7 and because we now
require Python 3.7+, we can drop support for aiohttp 3.0, 3.1, and 3.2.

…pport-python-3.10-and-aiohttp-3.8

aioresponses works with Python 3.11 with no issues so we update the
package metadata to say that, and we update the list of test
environments to include Python 3.11. Python 3.11 is supported by aiohttp
starting from its v3.8 ([1]), and we also need newer `yarl` ([2]).

[1] aio-libs/aiohttp#6600
[2] aio-libs/yarl#706