docs(flake8): Use src directory

Author: tony

setup.cfg

@@ -1,5 +1,5 @@
 [flake8]
-exclude = .*/,*.egg,libtmux/_compat.py,libtmux/__*__.py
+exclude = .*/,*.egg,src/libtmux/_compat.py,src/libtmux/__*__.py
 select = E,W,F,N
 max-line-length = 88
 # Stuff we ignore thanks to black: https://github.com/ambv/black/issues/429