CI, docs: Install test/py/requirements.txt as well

As noted by Quentin, in CI we should be at least versioning the pytest
that we install. To avoid problems later, go with the whole requirements
file being used. Furthermore, our documentation building for readthedocs
must also have pytest so install the requirements file there as well.

Reported-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Tom Rini <trini@konsulko.com>

.readthedocs.yml

@@ -22,3 +22,4 @@ formats: []
 python:
   install:
     - requirements: doc/sphinx/requirements.txt
+    - requirements: test/py/requirements.txt