Rdflib-sqlalchemy

Fixes https://github.com/RDFLib/rdflib-sqlalchemy/issues/92. Thanks, gjhiggins!

Fixes
-----
- Removing support for deprecated `rdflib.term.Statement`
- Making a more accurate count for store length
- Fixing query selection from query triple (87)
- Limiting index size for MySQL tables (67)

Maintenance
-----------
- Switching to Github Actions
- Adding automated deployment of development snapshots to PyPI. Only intended
for use in continuous integration environments

Features
--------

* Adding `max_terms_per_where` option allowing to limit the number of
terms in generated SQL in a call to addN

Fixes
-----

* Restoring the setuptools RDFLib store plugin entrypoint so users no
longer need to call `registerplugins` in most cases

* Making `addN`, `add`, `bind` not error out when there's a conflicting
entry

Maintenance
-----------

* Removing uses of rdflib.py3compat

* Adding configs for generating documentation

* Minor documentation cleanup

* Supporting Python 3.6 and 3.7