tox: add format target for code formatting

And verify the formatting in the lint target as well.

Fixes #14.

Author: radhermit

tox.ini

@@ -95,12 +95,24 @@ deps =
     cython-lint
     isort
     pylint
+    black
 commands =
     bash -c "shopt -s extglob globstar nullglob; cython-lint --max-line-length 100 **/*.p{yx,xd}"
     # check for improperly formatted imports in non-generated files
     isort -l 100 -s pkgcraft_c.pxd --check-only --diff benches examples src tests setup.py
     # check for unused imports
     pylint --score=no --disable=all --enable=W0611 benches examples tests setup.py
+    # verify formatting
+    black -l 100 --check --include "benches|examples|tests|setup.py" .
+
+[testenv:format]
+description = auto-format code using various procedures
+deps =
+    isort
+    black
+commands =
+    isort -l 100 -s pkgcraft_c.pxd benches examples src tests setup.py
+    black -l 100 --include "benches|examples|tests|setup.py" .
 
 [testenv:sdist]
 description = build the source distribution