Make deps directory persistent over upgrades (#7801)

* Use pip install --user if venv not active

* Set PYTHONUSERBASE to deps directory, when installing with --user
  option.
* Reset --prefix option to workaround incompatability when installing
  with --user option. This requires pip version 8.0.0 or greater.
* Require pip version 8.0.3.
* Do not delete deps directory on home assistant upgrade.
* Fix local lib mount and check package exist.

* Update and add tests

* Fix upgrade from before version 0.46

* Extract function to get user site

* Add function(s) to package util to get user site.
* Use async subprocess for one of the functions to get user site.
* Add function to package util to check if virtual environment is
  active.
* Add and update tests.

* Update version for last removal of deps dir

* Address comments

* Rewrite package util tests with pytest

* Rewrite all existing unittest class based tests for package util as
  test functions, and capitalize pytest fixtures.
* Add test for installing with target inside venv.

setup.py

@@ -18,7 +18,7 @@ REQUIRES = [
     'requests==2.14.2',
     'pyyaml>=3.11,<4',
     'pytz>=2017.02',
-    'pip>=7.1.0',
+    'pip>=8.0.3',
     'jinja2>=2.9.5',
     'voluptuous==0.10.5',
     'typing>=3,<4',