readthedocs: quote python version

Signed-off-by: Unai Martinez-Corral <umartinezcorral@antmicro.com>

.readthedocs.yml

diff --git a/.readthedocs.yml b/.readthedocs.yml
index 4f5e297..0b154b6 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -14,7 +14,7 @@
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
+  version: "3.7"
   install:
     - requirements: docs/requirements.txt