Fix python 3.9 and sqlalchemy 1.4 deprecations

[jtniehof]

This PR fixes deprecations for Python 3.9 (some of which would be errors in 3.10) and for sqlalchemy 1.4. Maintains support on earlier Python and sqlalchemy. Closes #83.

For Python 3.9/3.10:

• imports abstract base classes from collections.abc if possible (fallback to collections)
• Replaces is string comparisons with ==
• A little bit of style cleanup in directly-related lines

For sqlalchemy 1.4:

• table_names() was removed (affects tests only)
• mapped_table was replaced with persist_selectable (affects tests only)
• Make a subquery into an explicit selection (DButils)
• Make a subquery into an explicitly scalar subquery (newestVersionProblemFinder, which is untested)

This supersedes #84, which does not fix all warnings in #83 and didn't preserve namespacing on collections.

PR Checklist

• Pull request has descriptive title
• Pull request gives overview of changes
• New code has inline comments where necessary
• (N/A) Any new modules, functions or classes have docstrings consistent with dbprocessing style
• (N/A) Major new functionality has appropriate Sphinx documentation
• (N/A) Added an entry to release notes if fixing a major bug or providing a major new feature
• (see below) New features and bug fixes should have unit tests
• Relevant issues are linked in the description (use Closes # if this PR closes the issue, or some other reference, such as See # if it is related in some other way)

We don't yet have unit tests to make sure warnings aren't raised, and I didn't add them. But the tests now pass without warnings on Python 3.10 and sqlalchemy 1.4.

[dnadeau-lanl]

Awesome 😄