.flake8: Exclude venv and dotfiles when running flake8 from the command line

With pre-commit, flake8 is automatically run only on the diff to be
committed. However, when invoking `flake8` from the command line, the
current config covers all python files found in the entire repository,
including third-party packages in venv.

This patch teaches flake8 to exclude the venv directory as well as any
dotfiles when run from the command line.

Author: jherland

.flake8

@@ -1,5 +1,6 @@
 [flake8]
+exclude = .??*, venv
 ignore = E203, E266, E501, W503, F403, F401
 max-line-length = 88
 max-complexity = 18
-select = B,C,E,F,W,T4,B9
+select = B,C,E,F,W,T4,B9