Add feature flip for always upgrading user defined packages on pip install (#3201)

* Add feature flipping models to Projects, plus example feature flip

Also, use a new pattern for API instance objects

* Refactoring the make_api_* calls into the API models

* Add migrations for proxy models

* Add some admin features

* Bump setuptools version.

* Add feature flip for setuptools latest version

Also, add a convenience method on the Project model for feature flag dependent
values.

* Use `--upgrade` instead of `-U` in `pip` args

* Add upgrade flag to user environment pip install

Installs the user-provided requirements with the `--upgrade` flag.

* 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

* Rework feature relationship, add default true value based on date

This allows for features on deprecated code, where we can say the feature is
true historically.

* Fix some issues, more tests

* Rename Feature.feature -> Feature.feature_id, other cleanup

* Drop setuptools pinning

* Use semver for setuptools version

* Missed merge conflict

Author: agjohnson

readthedocs/doc_builder/python_environments.py

@@ -152,7 +152,7 @@ def install_core_requirements(self):
             self.venv_bin(filename='pip'),
             'install',
             '--use-wheel',
-            '-U',
+            '--upgrade',
             '--cache-dir',
             self.project.pip_cache_path,
         ]
@@ -182,14 +182,21 @@ def install_user_requirements(self):
                         break
 
         if requirements_file_path:
-            self.build_env.run(
+            args = [
                 'python',
                 self.venv_bin(filename='pip'),
                 'install',
+            ]
+            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

@@ -1003,10 +1003,12 @@ 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')),
     )
 
     projects = models.ManyToManyField(