Update codebase to Python >= 3.6 (#2612)

- Replaced some `.format` calls with f-strings.
- Removed `'r'` mode argument for `open` function as it is uneeded.
- Removed `__future__` import and `coding: utf-8` comment from a file.

setup.py

@@ -40,7 +40,8 @@ if sys.argv[-1] == 'publish':
     os.system("python setup.py sdist bdist_wheel")
     os.system("twine upload dist/*")
     print("You probably want to also tag the version now:")
-    print("  git tag -a {0} -m 'version {0}'".format(get_version("mkdocs")))
+    version = get_version("mkdocs")
+    print(f"  git tag -a {version} -m 'version {version}'")
     print("  git push --tags")
     sys.exit()