Refine flake8 excludes

Remove old things from the repo development history. The new list of
names reflects current development practices and files generated by
the repo.

.flake8

@@ -1,4 +1,4 @@
 [flake8]
-exclude = .git,.tox,__pycache__,.eggs,dist,venv,.venv*,venv27,virtualenv,docs,docs-source,build
+exclude = .git,.tox,__pycache__,.eggs,dist,.venv*,docs,build
 max-line-length = 88
 extend-ignore = W503,W504,E203