Add typing + mypy

This project's python code should be typed, and type checking should be part of the CI pipeline. I propose using mypy, we can look at pytest for an example of how to integrate it.

I'm going to try and add in typing. I'll perform changes on a per module level so the reviews are smaller and close this issue when the typing effort is complete. I'll update this issue as I progress

We'd need to do the following subtasks:

• Add mypy to our project: done in https://github.com/pytest-dev/pytest-html/pull/435
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/__init__.py: done in https://github.com/pytest-dev/pytest-html/pull/435
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/extras.py: done in https://github.com/pytest-dev/pytest-html/pull/435
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/hooks.py
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/html_report.py
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/outcome.py: done in https://github.com/pytest-dev/pytest-html/pull/435
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/plugin.py
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/result.py
• Type https://github.com/pytest-dev/pytest-html/blob/master/src/pytest_html/util.py: done in https://github.com/pytest-dev/pytest-html/pull/435
• Type https://github.com/pytest-dev/pytest-html/blob/master/testing/test_pytest_html.py