tox.ini: Fix paths in flake8 target

Author: mgorny

tox.ini

@@ -40,7 +40,7 @@ deps=
     flake8
     mccabe
 commands =
-    flake8 setuptools_scm/ testing/ setup.py --exclude=setuptools_scm/win_py31_compat.py
+    flake8 src/setuptools_scm/ testing/ setup.py --exclude=setuptools_scm/win_py31_compat.py
 
 [testenv:check_readme]
 skip_install=True