Update Travis config for default Python 2.7=>3.6 (mkdocs#1782)

Travis has changed their default Python version from 2.7 to 3.6 on April
16, 2019 in anticipation of PY27 support being dropped in 2020. 

We are now running flake8 on Python 3.

Unfortunately, LinkChecker does not yet support Python 3. We need to
specify Python 2.7 in both the travis and tox configs to get the test to 
pass. We'll remove the 2.7 requirement as soon as LinkChecker is updated.

Author: waylan

.travis.yml

@@ -9,6 +9,7 @@ matrix:
     - env: TOXENV=markdown-lint
       before_install: npm install -g markdownlint-cli
     - env: TOXENV=linkchecker
+      python: '2.7'
     - env: TOXENV=jshint
       before_install: npm install -g jshint
     - env: TOXENV=csslint

mkdocs/tests/cli_tests.py

@@ -49,7 +49,7 @@ def test_serve_config_file(self, mock_serve):
         if PY3:
             self.assertIsInstance(kwargs['config_file'], io.BufferedReader)
         else:
-            self.assertTrue(isinstance(kwargs['config_file'], file))
+            self.assertTrue(isinstance(kwargs['config_file'], file))  # noqa: F821
         self.assertEqual(kwargs['config_file'].name, 'mkdocs.yml')
 
     @mock.patch('mkdocs.commands.serve.serve', autospec=True)
@@ -227,7 +227,7 @@ def test_build_config_file(self, mock_build, mock_load_config):
         if PY3:
             self.assertIsInstance(kwargs['config_file'], io.BufferedReader)
         else:
-            self.assertTrue(isinstance(kwargs['config_file'], file))
+            self.assertTrue(isinstance(kwargs['config_file'], file))  # noqa: F821
         self.assertEqual(kwargs['config_file'].name, 'mkdocs.yml')
 
     @mock.patch('mkdocs.config.load_config', autospec=True)
@@ -405,7 +405,7 @@ def test_gh_deploy_config_file(self, mock_gh_deploy, mock_build, mock_load_confi
         if PY3:
             self.assertIsInstance(kwargs['config_file'], io.BufferedReader)
         else:
-            self.assertTrue(isinstance(kwargs['config_file'], file))
+            self.assertTrue(isinstance(kwargs['config_file'], file))  # noqa: F821
         self.assertEqual(kwargs['config_file'].name, 'mkdocs.yml')
 
     @mock.patch('mkdocs.config.load_config', autospec=True)

mkdocs/utils/ghp_import.py

@@ -47,12 +47,12 @@ def write(pipe, data):
                 raise
 else:
     def enc(text):
-        if isinstance(text, unicode):
+        if isinstance(text, unicode):  # noqa: F821
             return text.encode('utf-8')
         return text
 
     def dec(text):
-        if isinstance(text, unicode):
+        if isinstance(text, unicode):  # noqa: F821
             return text
         return text.decode('utf-8')
 

tox.ini

@@ -16,7 +16,6 @@ commands=
     py{27,34,35,36,37,py,py3}-integration: {envpython} -m mkdocs.tests.integration --output={envtmpdir}/builds
 
 [testenv:flake8]
-basepython = python2.7
 deps=-rrequirements/test.txt
 commands={envbindir}/flake8 mkdocs --max-line-length=119