Move docs to 3.8

This is the latest version currently supported by RTD.

.readthedocs.yml

@@ -2,7 +2,7 @@
 version: 2
 python:
   # Keep version in sync with tox.ini (docs and gh-actions).
-  version: 3.7
+  version: 3.8
 
   install:
     - method: pip

tox.ini

@@ -13,8 +13,8 @@ python =
     2.7: py27
     3.5: py35
     3.6: py36
-    3.7: py37, docs
-    3.8: py38, manifest, typing, changelog
+    3.7: py37
+    3.8: py38, manifest, typing, changelog, docs
     3.9: py39, pyright
     3.10: py310, lint
     pypy2: pypy
@@ -93,7 +93,7 @@ commands =
 
 [testenv:docs]
 # Keep basepython in sync with gh-actions and .readthedocs.yml.
-basepython = python3.7
+basepython = python3.8
 extras = docs
 commands =
     sphinx-build -n -T -W -b html -d {envtmpdir}/doctrees docs docs/_build/html