Force RTD to use py3.8

RTD was still using py3.7, which on its own probably would have been
okay. However, pybids/bids-validator now has a minimum python
requirement of py3.8, which I suspect made some changes to
`functools.lru_cache`. These changes caused RTD to error out when trying
to read the `bids-validator` code.

.readthedocs.yml

@@ -8,7 +8,7 @@ version: 2
 build:
   os: "ubuntu-22.04"
   tools:
-    python: "3.7"
+    python: "3.8"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: