Update the installation docs to use pip

I don't use pipenv and think it is less clear/popular than pip.

docs/tutorials/installation.rst

@@ -4,20 +4,8 @@ Installation
 ============
 
 Hypercorn is only compatible with Python 3.8 or higher and can be
-installed using pipenv or your favorite python package manager.
+installed using pip or your favorite python package manager.
 
 .. code-block:: sh
 
-    pipenv install hypercorn
-
-It is sufficient to run this single command in your working directory. Besides
-installing dependency, it will also create a Pipfile if one doesn't exist yet
-along with a linked virtualenv. Now you'll be able to activate your virtualenv
-using:
-
-.. code-block:: sh
-
-    pipenv shell
-
-To learn more about it visit `pipenv docs
-<https://docs.pipenv.org/install/#installing-packages-for-your-project>`_
+    pip install hypercorn