correct python version for RTD

Author: askpatrickw

{{ cookiecutter and 'tmp_repo' }}/{% if cookiecutter.sphinx_docs in ['y', 'yes'] %}.readthedocs.yaml{% endif %}

@@ -9,7 +9,7 @@
 version: 2
 
 python:
-  version: "3.x"
+  version: "3.7"
   install:
     - requirements: docs/requirements.txt
     - requirements: requirements.txt