Tests: pathlib used where appropriate

Also fixed various pylint errors and style issues.

test/test_python_isolation.py

@@ -1,9 +1,9 @@
-import os
 import re
 import subprocess
 from pathlib import Path
 
 import pytest
+
 from unit.applications.lang.python import ApplicationPython
 from unit.option import option
 from unit.utils import findmnt
@@ -24,10 +24,10 @@ def get_cgroup(app_name):
 
     cgroup = f'/proc/{pid}/cgroup'
 
-    if not os.path.isfile(cgroup):
+    if not Path(cgroup).is_file():
         pytest.skip(f'no cgroup at {cgroup}')
 
-    with open(cgroup, 'r') as f:
+    with open(cgroup, 'r', encoding='utf-8') as f:
         return f.read().rstrip()