Only run pyflakes on ndindex/ in the rever script

rever.xsh

@@ -21,7 +21,7 @@ def run_tests():
                            'mkl']):
         # Until numpy 1.20 is out, the tests require the git version to run
         pip install git+https://github.com/numpy/numpy.git
-        pyflakes .
+        pyflakes ndindex
         python -We:invalid -We::SyntaxWarning -m compileall -f -q ndindex/
         ./run_doctests
         pytest