CI: pin pylint

Pin pylint version to workaround pylint-dev/pylint#3524

Author: jon-turney

.github/workflows/lint_mypy.yml

@@ -19,7 +19,7 @@ jobs:
     - uses: actions/setup-python@v1
       with:
         python-version: '3.x'
-    - run: python -m pip install pylint
+    - run: python -m pip install pylint==2.4.4
     - run: pylint mesonbuild
 
   mypy: