Fix black exclusions to omit .gitignore dirs

This replaces "exclude" with "extend-exclude" in the black
configuration, so that it keeps its default exclusions, of which
all directories listed in .gitignore are automatically a part.
That makes it possible to run "black ." to format just the files
that should be formatted (git/ files, test/ files, and setup.py),
while automatically omitting .venv/, .tox/, build/, and so on.

This commit does not change how black is run yet, it just fixes
the way its exclusions are configured.

Author: EliahKagan

pyproject.toml

@@ -45,4 +45,4 @@ omit = ["*/git/ext/*"]
 [tool.black]
 line-length = 120
 target-version = ['py37']
-exclude = "git/ext/gitdb"
+extend-exclude = "git/ext/gitdb"