Bump oldest supported Python version to 3.7

[harripj]

Description of the change

As discussed in #343 Python 3.6 is deprecated, but currently used in CI as the oldest supported version. This is preventing implementation of type hinting (as discussed in #338) into the code base as annotations from __future__ is only available in Python 3.7+.

In this PR the oldest Python version used in CI is bumped to 3.7. A guide for implementing type hints is introduced into the contributing guide.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [n/a] Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• [n/a] New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in
  .zenodo.json.

[hakonanes]

This looks very good to me, just a minor point about the changelog.

I suggest to wait to merge this till tomorrow after the new branches develop (current master) and main are created. (See #243.)

[hakonanes]

Note that I've renamed the "master" branch to "develop". I hope this doesn't cause any surprises for you @harripj.

[hakonanes]

This documentation might be useful.

[harripj]

This PR is now pointing at the develop branch.

[hakonanes]

I'll merge after getting a cup of coffee ☕