Use pip<20.3 to fix ReadTheDocs builds (#5052)

.readthedocs.yml

@@ -12,5 +12,6 @@ python:
   # https://docs.readthedocs.io/en/stable/config-file/v2.html#build-image
   version: 3.8  # Keep in sync with .github/workflows/checks.yml
   install:
+    - requirements: docs/pip.txt
     - requirements: docs/requirements.txt
     - path: .

docs/pip.txt

@@ -0,0 +1,3 @@
+# In pip 20.3-21.0, the default dependency resolver causes the build in
+# ReadTheDocs to fail due to memory exhaustion or timeout.
+pip<20.3