Drop Python 2.7 support

[slott56]

Now that version 3.0 is available, can we drop support for Python 2.7? This would remove a some complications, and permit using type hints as well as making LatexNodeList a proper list[LatexNode] subclass.

Keeping compatibility with Python 3.8 is challenging, but, it doesn't look like the new type statement or match-case are required. (Jumping to Python 3.10 has numerous advantages language-wise, but the user community may have a large number of people who can't or won't upgrade.)