Attempt to fix compilation of documentation on readthedocs.org

By chance I noticed that several "pages" are empty on rtd.org, because
the Sphinx build fails because the pglast itself is not installed in the
virtual environment.

.readthedocs.yaml

@@ -15,5 +15,11 @@ build:
   tools:
     python: "3.11"
 
+python:
+  install:
+    - requirements: requirements.txt
+    - method: pip
+      path: .
+
 sphinx:
   configuration: docs/conf.py