Add feature flip for always upgrading user defined packages on pip install

We're not yet sure if this will blow up user defined requirements, feature here
will be used to test on a sample.

Refs #3077
Closes #3077

Author: agjohnson

readthedocs/doc_builder/python_environments.py

@@ -178,15 +178,21 @@ def install_user_requirements(self):
                         break
 
         if requirements_file_path:
-            self.build_env.run(
+            args = [
                 'python',
                 self.venv_bin(filename='pip'),
                 'install',
-                '--upgrade',
+            ]
+            if self.project.has_feature(Feature.PIP_ALWAYS_UPGRADE):
+                args += ['--upgrade']
+            args += [
                 '--exists-action=w',
                 '--cache-dir',
                 self.project.pip_cache_path,
                 '-r{0}'.format(requirements_file_path),
+            ]
+            self.build_env.run(
+                *args,
                 cwd=self.checkout_path,
                 bin_path=self.venv_bin()
             )

readthedocs/projects/models.py

@@ -991,10 +991,12 @@ def add_features(sender, **kwargs):
     # may be added by other packages
     USE_SPHINX_LATEST = 'use_sphinx_latest'
     USE_SETUPTOOLS_LATEST = 'use_setuptools_latest'
+    PIP_ALWAYS_UPGRADE = 'pip_always_upgrade'
 
     FEATURES = (
         (USE_SPHINX_LATEST, _('Use latest version of Sphinx')),
         (USE_SETUPTOOLS_LATEST, _('Use latest version of setuptools')),
+        (PIP_ALWAYS_UPGRADE, _('Always run pip install --upgrade')),
     )
 
     project = models.ForeignKey(