Drop Python 3.6 support, add Python 3.10 support

[dhimmel]

See https://discuss.python.org/t/python-3-6-rides-into-the-sunset/12964:

Python 3.6 which officially reached end-of-life on 2021-12-23

[dhimmel]

Hitting the following issue when testing in pytest manubot/webpage/tests/test_webpage_command.py::test_webpage_command:

OpenTimestamp command returned nonzero code (-6).
>>> ots stamp webpage/v/testing/index.html

Only happening on macOS, not linux or windows.

Edit: tracked in #320. We're currently skipping the part of the test that checks for OTS success on macOS.

[agitter]

@dhimmel should we update this line when we change supported Python versions?

manubot/pyproject.toml

Line 10 in 90925ee

target-version = ['py36', 'py37', 'py38']

[dhimmel]

should we update this line when we change supported Python versions?

Yes. Done in 3551dec.