update linting requirements

Author: Mikaayenson

setup.py

@@ -25,10 +25,12 @@
     "mock~=1.3.0",
     "pytest~=3.8.2",
     "pytest-cov==2.4",
-    "flake8==2.5.1",
+    "flake8==2.5.1; python_version<'3.8'",
+    "flake8>=3.8.0; python_version>='3.8'",
     "pep257==0.7.0",
     "coverage==4.5.3",
-    "flake8-pep257==1.0.5",
+    "flake8-pep257==1.0.5; python_version<'3.8'",
+    "flake8-docstrings>=1.5.0; python_version>='3.8'",
     "PyYAML<6.0; python_version<'3.4'",
     "PyYAML; python_version>='3.4'",
     "toml~=0.10",