Quote quard version requirements for pip installs

* As pointed out by Henry in PR 1216, adding quotes to version
  information on packages provides safeguards against common
  mistakes. Additionally this matches style with
  pypa/pip#11842.

source/guides/installing-using-pip-and-virtual-environments.rst

@@ -245,27 +245,27 @@ a specific version of ``requests``:
 
  .. code-block:: bash
 
- python3 -m pip install requests==2.18.4
+ python3 -m pip install 'requests==2.18.4'
 
 .. tab:: Windows
 
  .. code-block:: bat
 
- py -m pip install requests==2.18.4
+ py -m pip install "requests==2.18.4"
 
 To install the latest ``2.x`` release of requests:
 
 .. tab:: Unix/macOS
 
  .. code-block:: bash
 
- python3 -m pip install requests>=2.0.0,<3.0.0
+ python3 -m pip install 'requests>=2.0.0,<3.0.0'
 
 .. tab:: Windows
 
  .. code-block:: bat
 
- py -m pip install requests>=2.0.0,<3.0.0
+ py -m pip install "requests>=2.0.0,<3.0.0"
 
 To install pre-release versions of packages, use the ``--pre`` flag: