python-common/tox.ini: enable black formatting

Initially this is only for a few smaller packages/files that I don't
need to wrestle with to make it work with black.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

src/python-common/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = lint, rstcheck, mypy, py3
+envlist = lint, rstcheck, mypy, py3, check-black
 skip_missing_interpreters = true
 
 [testenv:py3]
@@ -36,3 +36,21 @@ deps =
     rstcheck
 commands =
     rstcheck --report-level info  README.rst
+
+
+# OPT-IN formatting with 'black'
+#  add your module to the modules list below to use automated formatting
+[black]
+deps = black>=23,<25
+options = -l78 -t py36 --skip-string-normalization
+modules = ceph/cephadm ceph/cryptotools ceph/fs ceph/utils.py
+
+[testenv:check-black]
+deps = {[black]deps}
+commands =
+    black --check -q {[black]options} {[black]modules}
+
+[testenv:format-black]
+deps = {[black]deps}
+commands =
+    black {[black]options} {[black]modules}