Use explicit requirements.txt file

See actions/setup-python#807
The alternative (creating an empty file) seems worse than using an actual requirements file.

Author: flodolo

.github/requirements.txt

@@ -0,0 +1 @@
+moz-fluent-linter~=0.4.9

.github/workflows/fluent_linter.yml

@@ -27,7 +27,7 @@ jobs:
           cache: 'pip'
       - name: Install Python dependencies
         run: |
-          pip install moz-fluent-linter==0.4.9
+          pip install -r .github/requirements.txt
       - name: Lint reference
         run: |
           moz-fluent-lint ./locales/en-US --config .github/fluent_linter_config.yml