Don't preinstall dependencies in test_installation

This removes the step in test_installation that did the equivalent
of "pip install -r requirements.txt", because installing GitPython
is sufficient to install all its required dependencies, and it is
more important to test that than to test requirements.txt directly.

Removing this causes the test to fail if installing the project
doesn't entail installation of the requirements necessary to import
the git module or to cause gitdb to be found in a sys.path search.

Author: EliahKagan

Diff for: test/test_installation.py

@@ -23,16 +23,6 @@ def setUp_venv(self, rw_dir):
     @with_rw_directory
     def test_installation(self, rw_dir):
         self.setUp_venv(rw_dir)
-        result = subprocess.run(
-            [self.pip, "install", "-r", "requirements.txt"],
-            stdout=subprocess.PIPE,
-            cwd=self.sources,
-        )
-        self.assertEqual(
-            0,
-            result.returncode,
-            msg=result.stderr or result.stdout or "Can't install requirements",
-        )
         result = subprocess.run(
             [self.pip, "install", "."],
             stdout=subprocess.PIPE,