test_serving: Use sys.executable, not 'python'
test_serving: Use sys.executable, not 'python'

This test failed on systems where the Python command is not called "python".
Also, it could execute Python 2 when pytest is run under py3, or vice versa.
fkazimierczak/werkzeug,mitsuhiko/werkzeug,pallets/werkzeug,pallets/werkzeug,pallets/werkzeug,fkazimierczak/werkzeug,mitsuhiko/werkzeug,fkazimierczak/werkzeug