ci: Use python -m pip to upgrade pip

Invoking pip directly will fail on Windows.

.github/workflows/docs.yml

@@ -31,7 +31,7 @@ jobs:
         sudo apt-get install -y libgettextpo-dev
     - name: Install dependencies
       run: |
-        pip install --upgrade pip wheel
+        python -m pip install --upgrade pip wheel
         pip install -r requirements/dev.txt
     - name: Build docs
       run: make docs

.github/workflows/setup.yml

@@ -32,7 +32,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install -y libgettextpo-dev
     - name: Update pip
-      run: pip install --upgrade pip wheel
+      run: python -m pip install --upgrade pip wheel
     - name: Install pip dependencies
       run: |
         pip install -r requirements/dev.txt

.github/workflows/test.yml

@@ -51,7 +51,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install -y libgettextpo-dev libxml2-dev libxmlsec1-dev gettext hunspell-af
     - name: Update pip
-      run: pip install --upgrade pip wheel
+      run: python -m pip install --upgrade pip wheel
     - name: Install pip dependencies
       run: pip install -r requirements/test.txt
     - name: Install pip dependencies