Install the latest coverage version; this 'fixes' the test source problem. [skip appveyor]

By not actually recording anything for the tests. Presumably because they're executed as scripts, not as gevent.tests.whatever.

setup.py

@@ -373,7 +373,7 @@ def run_setup(ext_modules, run_make):
 
                 # We don't run coverage on Windows, and pypy can't build it there
                 # anyway (coveralls -> cryptopgraphy -> openssl)
-                'coverage>=4.0 ; sys_platform != "win32"',
+                'coverage>=5.0a3 ; sys_platform != "win32"',
                 'coveralls>=1.0 ; sys_platform != "win32"',
 
                 'futures ; python_version == "2.7"',