Stay on pylint 2.x.x for tests

pylint 3.x.x brings significant changes, so current PRs may fail or even break on tests.
One of the cases that happened in ranger#2911 with pylint 3.0.1:
```
File "ranger/tests/pylint/python3.py", line 184, in Python3Checker
    __implements__ = interfaces.IAstroidChecker
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'pylint.interfaces' has no attribute 'IAstroidChecker'
make: *** [test_pylint] Error 1
```

requirements.txt

@@ -1,3 +1,3 @@
 flake8
-pylint
+pylint==2.17.7
 pytest