style: add minimal black-compatible flake8 configuration

I have enjoyed used black for manually-triggered code reformatting,
and flake8 to catch some things, including e.g. unused imports. This
will help surpress warnings that aren't useful.

Note: running black or flake8 is left to individual developers, and is
not done automatically.

setup.cfg

@@ -58,3 +58,7 @@ show_missing = True
 omit =
     # versioneer code
     datalad_redcap/_version.py
+
+[flake8]
+max-line-length = 88
+extend-ignore = E203