Remove lxml

[vkbo]

Summary:

This PR:

• Replaces lxml with Python standard library xml package in the Project XML read/write classes.
• Replaces lxml with Python standard library xml package in the ODT writer class.
• Adds a custom xml indent function based on the one from the standard library. This one behaves more closely to how the indentation from lxml was added. In particular, it allows inline elements that do not have line breaks by explicitly requiring that text and tail attributes are set to None before breaks are added.
• Update all test and test reference files.
• Drop CI for Python 3.7, but Python 3.7 support itself is not yet officially dropped. It will likely be dropped before 2.1 is released though.
• Remove all references to the lxml package in the source, install files, documentation, and tests.

Related Issue(s):

Closes #1257

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed