Add --ignore-requires-python to GitHub workflow pip install

Author: jsvine

.github/workflows/tests.yml

@@ -23,8 +23,8 @@ jobs:
     - name: Install Python dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -r requirements.txt
-        pip install -r requirements-dev.txt
+        pip install -r requirements.txt --ignore-requires-python
+        pip install -r requirements-dev.txt --ignore-requires-python
 
     - name: Validate against psf/black
       run: python -m black --check pdfplumber tests