Documentation feedback: Don't hard-code Python 3.7 for Black.

docs/how-to/develop-on-borgmatic.md

@@ -62,6 +62,8 @@ following:
 tox -e black
 ```
 
+Note that Black requires at minimum Python 3.6.
+
 And if you get a complaint from the
 [isort](https://github.com/timothycrosley/isort) Python import orderer, you
 can ask isort to order your imports for you:

tox.ini

@@ -19,7 +19,6 @@ commands =
     flake8 borgmatic tests
 
 [testenv:black]
-basepython = python3.7
 commands =
     black {posargs} .