Add configuration to run Black

Introduce two new tox environments - `format` and `format-check`. The
`format` environment will reformat the code using `black` whereas the
`format-check` envrionment will simply print the diff without modifying
any files.

Configure Black to use the same line lengths that we use
currently, i.e. 119.

1 files changed, 20 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index 4c18f2c14..d8fa3e984 100644
--- a/tox.ini
+++ b/tox.ini
@@ -103,6 +103,26 @@ setenv =
     COVERAGE_FILE = {toxinidir}/.coverage-reports/.coverage
 
 #
+# Code formatters
+#
+[testenv:format]
+skip_install = True
+deps =
+    black==19.10b0
+commands =
+    black {posargs: src tests}
+
+#
+# Code format checkers
+#
+[testenv:format-check]
+skip_install = True
+deps =
+    black==19.10b0
+commands =
+    black --check --diff {posargs: src tests}
+
+#
 # Running linters
 #
 [testenv:lint]