Fix doc build

Author: benjeffery

.github/workflows/docs.yml

@@ -16,7 +16,7 @@ env:
 jobs:
   build-deploy-docs:
     name: Docs
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       - name: Cancel Previous Runs
         uses: styfle/[email protected]
@@ -39,12 +39,15 @@ jobs:
         id: cache
         with:
           path: venv
-          key: docs-venv-v1-${{ hashFiles('requirements/CI-docs/requirements.txt') }}
+          key: docs-venv-v2-${{ hashFiles('requirements/CI-docs/requirements.txt') }}
 
       - name: Build virtualenv
         if: steps.cache.outputs.cache-hit != 'true'
         run: python -m venv venv
 
+      - name: Downgrade pip to allow conflicting deps
+        run: venv/bin/activate && pip install pip==20.0.2
+
       - name: Install deps
         run: venv/bin/activate && pip install -r requirements/CI-docs/requirements.txt
 

requirements/CI-docs/requirements.txt

@@ -13,3 +13,4 @@ tskit==0.5.3
 tskit-book-theme==0.3.2
 tqdm==4.64.0
 zarr==2.11.3
+jsonschema[format-nongpl]==4.17.3