Add black-compatible isort configuration.

lines_after_imports = 2 is a matter of taste; I like it.

setup.cfg

+[isort]
+ensure_newline_before_comments = True
+force_grid_wrap = 0
+include_trailing_comma = True
+known_first_party = transformers
+line_length = 119
+lines_after_imports = 2
+multi_line_output = 3
+use_parentheses = True