Drop unsupported pythons

[matthewhughes934]

Drop support for pythons <3.7 (including python 2).

Since this makes it part of the standard library, replace mock with
unittest.mock in tests. I also bumped the ubuntu version used in CI to
one that has python3.10 installable.

This also required replacing nose with pytest in tests This is
because nose is not under active development[1], and the version
specified fails to run on python3 (and newer versions fail to run on
python3.10[2]):

$ python -m nose
Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 187, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/usr/lib/python3.10/runpy.py", line 146, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File "/usr/lib/python3.10/runpy.py", line 110, in _get_module_details
    __import__(pkg_name)
  File "/home/mjh/src/pystatsd/.venv/lib/python3.10/site-packages/nose/__init__.py", line 1, in <module>
    from nose.core import collector, main, run, run_exit, runmodule
  File "/home/mjh/src/pystatsd/.venv/lib/python3.10/site-packages/nose/core.py", line 143
    print "%s version %s" % (os.path.basename(sys.argv[0]), __version__)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

pytest is pinned at the lowest version supporting python3.10[3],
similarly for coverage[4], flake8 was also bump to a more recent
version.

[1] https://nose.readthedocs.io/en/latest/#note-to-users
[2] nose-devs/nose#1122
[3] https://docs.pytest.org/en/7.1.x/changelog.html#pytest-6-2-5-2021-08-29
[4] https://coverage.readthedocs.io/en/6.4.1/changes.html#version-6-0-2021-10-03