'Install basic Read the Docs requirements into the virtualenv.'

def install_core_requirements(self):

    requirements = ['Pygments==2.2.0', 'setuptools==28.8.0', 'docutils==0.13.1', 'mock==1.0.1', 'pillow==2.6.1', 'alabaster>=0.7,<0.8,!=0.7.5', 'commonmark==0.5.4', 'recommonmark==0.4.0']
    if (self.project.documentation_type == 'mkdocs'):
        requirements.append('mkdocs==0.15.0')
    else:
        requirements.extend(['sphinx==1.5.3', 'sphinx-rtd-theme<0.3', 'readthedocs-sphinx-ext<0.6'])
    cmd = ['python', self.venv_bin(filename='pip'), 'install', '--use-wheel', '-U', '--cache-dir', self.project.pip_cache_path]
    if self.config.use_system_site_packages:
        cmd.append('-I')
    cmd.extend(requirements)
    self.build_env.run(bin_path=self.venv_bin(), *cmd)
