Set lint basepython to 3.7

3.7 is the lowest common denominator of supported versions. Linting on more
recent versions introduces new error messages.

Author: marcgibbons

tox.ini

@@ -70,6 +70,8 @@ commands =
     - sphinx-build -b html -b linkcheck -aEnQW doc doc/_build/html
 
 [testenv:lint]
+# PYVERSIONS
+basepython = python3.7
 deps =
     -r requirements/lint.pip