fix: type errors resulting from new mypy

[aucampia]

Summary of changes

New mypy version is reporting new errors.

In the long run we need to switch to poetry so we can better control this.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

This PR has no runtime changes:

$ PYLOGGING_LEVEL=INFO task run -- git difftool -y -x $(readlink -f devtools/diffrtpy.py) upstream/main | tee /var/tmp/compact.diff
task: [run] 
VIRTUAL_ENV="/home/iwana/sw/d/github.com/iafork/rdflib.reviews/.venv" PATH="/home/iwana/sw/d/github.com/iafork/rdflib.reviews/.venv/bin${PATH:+:${PATH}}" git difftool -y -x /home/iwana/sw/d/github.com/iafork/rdflib.reviews/devtools/diffrtpy.py upstream/main


2022-11-18T20:58:59.054 640638 139738907522880 030:WARNING  root         diffrtpy:40:clean_python failed to strip type hints from /tmp/git-blob-KQRDVq/algebra.py, falling back to using with type hints
2022-11-18T20:58:59.681 640638 139738907522880 030:WARNING  root         diffrtpy:40:clean_python failed to strip type hints from rdflib/plugins/sparql/algebra.py, falling back to using with type hints
--- a/setup.cfg
+++ b/setup.cfg
@@ -55,10 +55,11 @@
 warn_unused_configs = True
 ignore_missing_imports = True
 disallow_subclassing_any = False
 warn_unreachable = True
 warn_unused_ignores = True
+no_implicit_optional = False
 
 # This is here to exclude the setup.py files in test plugins because these
 # confuse mypy as mypy think they are the same module.
 exclude = (?x)(
     ^.*test/plugins/.*/setup.py$

[aucampia]

Merging as this has no changes that could affect users.