Pin jinja2 in docs requirements to keep readthedocs builds from failing (#1439)

* Attempt to pin jinja requirement to keep readthedocs from failing
* Change read the docs Python version from 3.7 to 3.10
* Unpin jinja and remove deprecated python version setting
* Try pinning to jinja 3.0.3
* Fix "jinja2" (instead of "jinja")

docs_theme/requirements.txt

@@ -1 +1,2 @@
 mkdocs==1.2.3
+jinja2==3.0.3 # https://github.com/readthedocs/readthedocs.org/issues/9037

readthedocs.yml

@@ -17,6 +17,5 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
     - requirements: docs_theme/requirements.txt