`ModuleNotFoundError: No module named 'six'` on build
## Details

I tried building my docs for my project, yet this fails with a ModuleNotFoundError

[My conf file](https://github.com/Arthurdw/dblstats/blob/v0.1.6/docs/source/conf.py)
[Build url](https://readthedocs.org/projects/dblstats/builds/12441143/)

## Expected Result

A successful build

## Actual Result

The following error:
```
Running Sphinx v3.3.1

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 319, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 89, in execfile_
    exec(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/checkouts/latest/docs/source/conf.py", line 82, in <module>
    from six import string_types
ModuleNotFoundError: No module named 'six'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 279, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 219, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 174, in read
    namespace = eval_config_file(filename, tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 332, in eval_config_file
    raise ConfigError(msg % traceback.format_exc()) from exc
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 319, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 89, in execfile_
    exec(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/checkouts/latest/docs/source/conf.py", line 82, in <module>
    from six import string_types
ModuleNotFoundError: No module named 'six'


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/config.py", line 319, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/envs/latest/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 89, in execfile_
    exec(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/dblstats/checkouts/latest/docs/source/conf.py", line 82, in <module>
    from six import string_types
ModuleNotFoundError: No module named 'six'
```
diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index c6fa55903..82273c823 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -367,6 +367,7 @@ class Virtualenv(PythonEnvironment):
             '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',
         ]
@@ -623,6 +624,7 @@ class Conda(PythonEnvironment):
         # Install pip-only things.
         pip_requirements = [
             'recommonmark',
+            'six',
         ]
 
         if self.config.doctype == 'mkdocs':
diff --git a/readthedocs/rtd_tests/tests/test_doc_building.py b/readthedocs/rtd_tests/tests/test_doc_building.py
index 5fc655a1f..15efe9b89 100644
--- a/readthedocs/rtd_tests/tests/test_doc_building.py
+++ b/readthedocs/rtd_tests/tests/test_doc_building.py
@@ -1197,6 +1197,7 @@ class TestPythonEnvironment(TestCase):
             'mock',
             'pillow',
             'alabaster',
+            'six',
         ]
         self.base_conda_requirements = [
             'mock',
@@ -1364,6 +1365,7 @@ class TestPythonEnvironment(TestCase):
         conda_requirements = self.base_conda_requirements + conda_sphinx
         pip_requirements = [
             'recommonmark',
+            'six',
             'readthedocs-sphinx-ext',
         ]
 
@@ -1405,6 +1407,7 @@ class TestPythonEnvironment(TestCase):
         conda_requirements = self.base_conda_requirements
         pip_requirements = [
             'recommonmark',
+            'six',
             'mkdocs',
         ]
 
['readthedocs/doc_builder/python_environments.py', 'readthedocs/rtd_tests/tests/test_doc_building.py']