feat: Upgrade pip before installing packages

As recommended by `pip install`.

Author: l0b0

.github/workflows/continuous-integration.yml

@@ -57,6 +57,7 @@ jobs:
 
       - name: Install dependencies
         run: |
+          pip install --upgrade pip
           pip install -r requirements-test.txt
           pip install -e ".[validation]"
 
@@ -100,6 +101,7 @@ jobs:
 
       - name: Install dependencies
         run: |
+          pip install --upgrade pip
           pip install -r requirements-test.txt
 
       - name: Execute linters & type checkers