Call mkdocs using -m

There are some cases were rtd uses the old installed version of mkdocs
instead of the one installed by the user, like in
readthedocs#5532 (comment)

Which gives an error because it's using a yaml file
supported only in new mkdocs versions.

This is similar to readthedocs#5107

readthedocs/doc_builder/backends/mkdocs.py

@@ -247,8 +247,9 @@ def generate_rtd_data(self, docs_dir, mkdocs_config):
     def build(self):
         checkout_path = self.project.checkout_path(self.version.slug)
         build_command = [
-            'python',
-            self.python_env.venv_bin(filename='mkdocs'),
+            self.venv_bin(filename='python'),
+            '-m',
+            'mkdocs',
             self.builder,
             '--clean',
             '--site-dir',