Fix undefined labels

- pip, Pipenv, Pipfile

docs/dev/virtualenvs.rst

@@ -47,7 +47,7 @@ install the latest 3.x version from `python.org`_ or refer to the
     `getting started tutorial`_ for an introduction to using your operating
     system's shell and interacting with Python.
 
-Additionally, you'll need to make sure you have :ref:`pip` available. You can
+Additionally, you'll need to make sure you have `pip`_ available. You can
 check this by running:
 
 .. code-block:: bash
@@ -60,6 +60,7 @@ using your OS package manager, you may have to `install pip <https://pip.pypa.io
 
 .. _getting started tutorial: https://opentechschool.github.io/python-beginners/en/getting_started.html#what-is-python-exactly
 .. _python.org: https://python.org
+.. _pip: https://pypi.org/project/pip/
 .. _Homebrew: https://brew.sh
 .. _Installing Python: http://docs.python-guide.org/en/latest/starting/installation/
 
@@ -67,9 +68,9 @@ using your OS package manager, you may have to `install pip <https://pip.pypa.io
 Installing Pipenv
 -----------------
 
-:ref:`Pipenv` is a dependency manager for Python projects. If you're familiar
+`Pipenv`_ is a dependency manager for Python projects. If you're familiar
 with Node.js' `npm`_ or Ruby's `bundler`_, it is similar in spirit to those
-tools. While :ref:`pip` can install Python packages, Pipenv is recommended as
+tools. While `pip`_ can install Python packages, Pipenv is recommended as
 it's a higher-level tool that simplifies dependency management for common use
 cases.
 
@@ -100,6 +101,7 @@ Use ``pip`` to install Pipenv:
     user ``PATH`` permanently in the `Control Panel`_. You may need to log
     out for the ``PATH`` changes to take effect.
 
+.. _Pipenv: https://docs.pipenv.org/
 .. _npm: https://www.npmjs.com/
 .. _bundler: http://bundler.io/
 .. _user base: https://docs.python.org/3/library/site.html#site.USER_BASE
@@ -120,11 +122,13 @@ tutorial) and run:
     $ pipenv install requests
 
 Pipenv will install the excellent `Requests`_ library and create a ``Pipfile``
-for you in your project's directory. The :ref:`Pipfile` is used to track which
+for you in your project's directory. The `Pipfile`_ is used to track which
 dependencies your project needs in case you need to re-install them, such as
 when you share your project with others. You should get output similar to this
 (although the exact paths shown will vary):
 
+.. _Pipfile: https://github.com/pypa/pipfile
+
 .. code-block:: text
 
     Creating a Pipfile for this project...