Prevent a Black/isort conflict

Comments in import blocks cause disagreement between Black and isort.
This will soon be fixed by specifying
'ensure_newline_before_comments=true' in the isort config, though this
won't be honoured until the next isort release.

See psf/black#251
and PyCQA/isort#1000

pyproject.toml

@@ -12,3 +12,4 @@ include_trailing_comma=true
 force_grid_wrap=0
 use_parentheses=true
 line_length=88
+ensure_newline_before_comments=true