Workaround flaky RTD behaviour in not putting the directory holding cython on the path. [skip ci]

setup.py

@@ -398,6 +398,12 @@ def run_setup(ext_modules, run_make):
 
 
 if __name__ == '__main__':
+    if os.getenv('READTHEDOCS'):
+        # Sometimes RTD fails to put our virtualenv bin directory
+        # on the PATH, meaning we can't run cython. Fix that.
+        new_path = os.environ['PATH'] + os.pathsep + os.path.dirname(sys.executable)
+        os.environ['PATH'] = new_path
+
     try:
         run_setup(ext_modules, run_make=run_make)
     except BuildFailed: