Drop support for Python 3.6, support Python 3.10

[tomaarsen]

Closes #2912, closes #2913

Hello!

Pull request overview

• Set python_requires in setup.py to >=3.7 instead of >=3.6.
• Removed mention of supporting Python 3.6 in documentation.
• Mention support for Python 3.10 in documentation.
• Removed py36 tests in tox, and add py310 tests.

---

Note

I left mentions of Python 3.6 in the tools folder, e.g. in run_doctests.py:

nltk/tools/run_doctests.py

Lines 10 to 16 in 6411fab

	for root, dirs, filenames in os.walk('.'):
	for filename in filenames:
	if filename.endswith('.py'):
	path = os.path.join(root, filename)
	for pyver in ["python3.5", "python3.6", "python3.7"]:
	print(pyver, filename, file=sys.stderr)
	subprocess.call([pyver, "-m", "doctest", path])

This folder seems mostly outdated and unused, also containing references back to jenkins and travis. One of the files, run_doctests.py, is not the proper way that doctests should be ran regardless. I didn't worry about these cases too much.

Thanks to @purificant for the commit of his I borrowed from his Python 3.10 branch :)

We should prepare a release soon.

• Tom Aarsen

[tomaarsen]

This PR is now planned to be merged after the upcoming NLTK release (3.6.6), allowing existing Python 3.6 users to benefit from our changes over the past months.

[stevenbird]

Merged, now that 3.6.6 is released.

[stevenbird]

Thanks @tomaarsen, @purificant

[tomaarsen]

Weirdly enough - these tests passed, but every single CI run after this PR fails to build scikit-learn on Windows. I've removed CI tests for Windows 10 for now, until scikit-learn 1.0.2 releases (planned in middle December 2021), which supposedly adds support for Python 3.10 in Windows.