Remove py2 import remnants, sort imports, cleanup, move to pytest, enable python 3.10

[ix5]

treewide: Use logger.warning, warn is deprecated

Fixes a pytest warning:

isso/__init__.py:187: DeprecationWarning: \
    The 'warn' method is deprecated, use 'warning' instead

See: https://docs.python.org/3.9/library/logging.html?highlight=logging#logging.Logger.warning

Note: There is an obsolete method warn which is functionally identical to warning.
As warn is deprecated, please do not use it - use warning instead.

Makefile: Run flakes explictly only inside isso/

This fixes an annoyance for people who have a virtualenv installed under e.g. .venv/ where flake8 would try to verify every single site-packages pip package inside.

tox: Check TOX_ENV_NAME to skip build_py js step

The env variable name seems to have changed in recent tox versions, see https://tox.wiki/en/latest/config.html#injected-environment-variables

tox.ini: Add python 3.10 to test env list

.github: pythonpackage: Add py3.10 to matrix

Note: The quotes around "py3.10" are needed since without them, GH Actions will interpret everything after the dot as a single-digit integer, i.e. "Python 3.1".

actions/setup-python#249 (comment)

setup.py: Add tests_require for local testing

Explicitly list pytest and pytest-cov deps as needed for local testing.

Note that the infinitve is used, not requires as in install_requires. That's just an oddity in setuptools.

treewide: Replace nosetests with pytest

Nosetests has long been abandoned by its authors and will no longer run under Python 3.10.
Pytest seems to be its spiritual successor as a test runner and seems widely accepted in the community.

• Adjust Makefile accordingly and recommend installing
  pytest, pytest-cov pip packages
• Adjust tox.ini accordingly
• Adjust releaser.conf accordingly
• Adjust releasing.rst accordingly
• Adjust docs accordingly
• Adjust GitHub Actions CI file accordingly

treewide: Remove py2 import remnants, sort

A lot of try/catch handling for discrepancy between py2/py3 module names as well as __future__ imports is not needed since Isso does not support py2 any longer.