minor #13209 speedup sphinx build with caching pip (smoench)

This PR was merged into the 3.4 branch.

Discussion
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speedup sphinx build with caching pip

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Commits
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65f9417 speedup sphinx build/linting with caching doctrees and pip cache

Author: OskarStark

.github/workflows/ci.yaml

@@ -29,10 +29,15 @@ jobs:
             -   name: "Install Sphinx dependencies"
                 run: sudo apt-get install python-dev build-essential
 
-            -   name: "Install Sphinx"
-                run: pip install --user sphinx
+            -   name: "Cache pip"
+                uses: actions/cache@v1
+                with:
+                    path: ~/.cache/pip
+                    key: ${{ runner.os }}-pip-${{ hashFiles('_build/.requirements.txt') }}
+                    restore-keys: |
+                        ${{ runner.os }}-pip-
 
-            -   name: "Install custom requirements via pip"
+            -   name: "Install Sphinx + requirements via pip"
                 run: pip install -r _build/.requirements.txt
 
             -   name: "Build documentation"