title: Sphinx docs builds fail on readthedocs for private GitHub repository
body: <p>I have set up and built Sphinx docs (<code>2.1.2</code>) for a Python 3.6 project which is currently in a private GitHub repository. I want to host the docs on readthedocs.org after releasing the project publicly, but at the moment the readthedocs builds are always failing with this error message</p>
  
  <pre><code> git clone --no-single-branch --depth 50 https://github.com/&lt;user name&gt;/&lt;repo name&gt; .
  
  Cloning into '.'...
  fatal: could not read Username for 'https://github.com': terminal prompts disabled
  </code></pre>
  
  <p>I've tried to look at the readthedocs project settings but I can't seem to find the right settings to modify to get the builds to work. The config. YAML was copied directly from the readthedocs template for Python projects.</p>
  
  <pre><code># .readthedocs.yml
  # Read the Docs configuration file
  # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
  
  # Required
  version: 2
  
  # Build documentation in the docs/ directory with Sphinx
  sphinx:
    builder: html
    configuration: docs/conf.py
    fail_on_warning: false
  
  # Build documentation with MkDocs
  #mkdocs:
  #  configuration: mkdocs.yml
  
  # Optionally build your docs in additional formats such as PDF and ePub
  formats: htmlzip
  
  # Optionally set the version of Python and requirements required to build your docs
  python:
     version: 3.6
     install:
        - requirements: docs/requirements.txt
        - requirements: requirements.txt
        - method: pip
     system_packages: true
  </code></pre>
  
  <p>This is <code>docs/conf.py</code>.</p>
  
  <pre><code># Configuration file for the Sphinx documentation builder.
  #
  # This file only contains a selection of the most common options. For a full
  # list see the documentation:
  # http://www.sphinx-doc.org/en/master/config
  
  # -- Path setup --------------------------------------------------------------
  
  # If extensions (or modules to document with autodoc) are in another directory,
  # add these directories to sys.path here. If the directory is relative to the
  # documentation root, use os.path.abspath to make it absolute, like shown here.
  #
  import os
  import sys
  sys.path.insert(0, os.path.abspath('.'))
  sys.path.insert(0, os.path.abspath(os.pardir))
  
  
  # -- Project information -----------------------------------------------------
  
  project = '&lt;project name&gt;'
  copyright = '2019, &lt;author&gt;'
  author = '&lt;author&gt;'
  
  # The full version, including alpha/beta/rc tags
  release = '0.1.0'
  
  
  # -- General configuration ---------------------------------------------------
  
  # Add any Sphinx extension module names here, as strings. They can be
  # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
  # ones.
  extensions = [
      'recommonmark',
      'sphinx.ext.autodoc',
      'sphinxcontrib.httpdomain',
      'sphinx.ext.intersphinx',
      'sphinx.ext.todo',
      'sphinx.ext.coverage',
      'sphinx.ext.viewcode',
      'sphinx.ext.githubpages'
  ]
  
  # Add any paths that contain templates here, relative to this directory.
  templates_path = ['_templates']
  
  # The suffix(es) of source filenames.
  # You can specify multiple suffix as a list of string:
  #
  source_suffix = ['.rst', '.md']
  
  # List of patterns, relative to source directory, that match files and
  # directories to ignore when looking for source files.
  # This pattern also affects html_static_path and html_extra_path.
  exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
  
  # The name of the Pygments (syntax highlighting) style to use.
  pygments_style = 'sphinx'
  
  # If true, `todo` and `todoList` produce output, else they produce nothing.
  todo_include_todos = True
  
  
  # -- Options for HTML output -------------------------------------------------
  
  # The theme to use for HTML and HTML Help pages.  See the documentation for
  # a list of builtin themes.
  #
  html_theme = 'alabaster'
  
  # Add any paths that contain custom static files (such as style sheets) here,
  # relative to this directory. They are copied after the builtin static files,
  # so a file named "default.css" will overwrite the builtin "default.css".
  html_static_path = ['_static']
  
  # Example configuration for intersphinx: refer to the Python standard library.
  intersphinx_mapping = {'https://docs.python.org/': None}
  </code></pre>