diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index f470966a15f..3ab079675ef 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -364,7 +364,6 @@ def install_core_requirements(self):
             'mock==1.0.1',
             'pillow==5.4.1',
             'alabaster>=0.7,<0.8,!=0.7.5',
-            'six',
             'commonmark==0.8.1',
             'recommonmark==0.5.0',
         ]
@@ -658,7 +657,6 @@ def _get_core_requirements(self):
         # Install pip-only things.
         pip_requirements = [
             'recommonmark',
-            'six',
         ]
 
         if self.config.doctype == 'mkdocs':
diff --git a/readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl b/readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl
index e2c5a4bf95f..dc3c8e11e9b 100644
--- a/readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl
+++ b/readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl
@@ -12,12 +12,15 @@
 # conf.py.tmpl file found in the readthedocs.org codebase:
 # https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl
 #
-
+# Note: this file should't rely on extra depencies.
 
 import importlib
 import sys
 import os.path
-from six import string_types
+
+# Borrowed from six.
+PY3 = sys.version_info[0] == 3
+string_types = str if PY3 else basestring
 
 from sphinx import version_info
 
diff --git a/readthedocs/rtd_tests/tests/test_doc_building.py b/readthedocs/rtd_tests/tests/test_doc_building.py
index 786ec6048ad..f67d2b65d4a 100644
--- a/readthedocs/rtd_tests/tests/test_doc_building.py
+++ b/readthedocs/rtd_tests/tests/test_doc_building.py
@@ -1194,7 +1194,6 @@ def setUp(self):
             'mock',
             'pillow',
             'alabaster',
-            'six',
         ]
         self.base_conda_requirements = [
             'mock',
@@ -1374,7 +1373,6 @@ def test_install_core_requirements_sphinx_conda(self, checkout_path):
         conda_requirements = self.base_conda_requirements + conda_sphinx
         pip_requirements = [
             'recommonmark',
-            'six',
             'readthedocs-sphinx-ext',
         ]
 
@@ -1416,7 +1414,6 @@ def test_install_core_requirements_mkdocs_conda(self, checkout_path):
         conda_requirements = self.base_conda_requirements
         pip_requirements = [
             'recommonmark',
-            'six',
             'mkdocs',
         ]