Ignore installed python packages in downstream CI

.github/workflows/downstream.yml

@@ -50,12 +50,16 @@ jobs:
           - name: "Clone libsemigroups_pybind11 . . ."
             run: git clone "$URL" --depth 1 --branch v1
           - name: "Install prerequisites for libsemigroups_pybind11 . . ."
+          # The flag --ignore-installed is included below so pip doesn't try and
+          # uninstall the system version of the packages specified in
+          # requirements.txt. This was happening with the typing_extensions
+          # package.
             run: |
               cd libsemigroups_pybind11
               sudo apt-get --yes update
               sudo apt-get install python3 --yes
               sudo apt-get install python3-pip --yes
-              sudo -H pip3 install -r requirements.txt
+              sudo -H pip3 install --ignore-installed -r requirements.txt
           - name: "Building libsemigroups_pybind11 . . ."
             run: |
               cd libsemigroups_pybind11