create better fixtures from the environ

[evgeni]

I think this could be done with a fixture:

@pytest.fixture
def temporary_environ():
  old_environ = os.environ.copy()
  yield
  os.environ = old_environ

Then on the test:

@pytest.mark.usefixtures("temporary_environ")
def test_init_with_existing_cachedir(fixture_dir, requests_mock, tmpdir):

I think you can even stack it:

@pytest.fixture
def temporary_xdg_cache_home(tmpdir):
    old_environ = os.environ.copy()
    os.environ['XDG_CACHE_HOME'] = tmpdir.strpath
    yield
    os.environ = old_environ

_Originally posted by @ekohl in #58