docs: never run pip directly

Author: nedbat

doc/contributing.rst

@@ -48,7 +48,7 @@ these steps:
 
 #.  Install the requirements::
 
-    $ pip install -r requirements/dev.pip
+    $ python3 -m pip install -r requirements/dev.pip
 
 #.  Install a number of versions of Python.  Coverage.py supports a range
     of Python versions.  The more you can test with, the more easily your code

doc/index.rst

@@ -56,7 +56,7 @@ Getting started is easy:
 
 #.  Install coverage.py::
 
-        $ pip install coverage
+        $ python3 -m pip install coverage
 
     For more details, see :ref:`install`.
 

doc/install.rst

@@ -15,19 +15,19 @@ Installation
 
 You can install coverage.py in the usual ways. The simplest way is with pip::
 
-    $ pip install coverage
+    $ python3 -m pip install coverage
 
 .. ifconfig:: prerelease
 
     To install a pre-release version, you will need to specify ``--pre``::
 
-        $ pip install --pre coverage
+        $ python3 -m pip install --pre coverage
 
     or the exact version you want to install:
 
     .. parsed-literal::
 
-        $ pip install |coverage-equals-release|
+        $ python3 -m pip install |coverage-equals-release|
 
 .. _install_extension:
 

doc/plugins.rst

@@ -29,7 +29,7 @@ a coverage.py plug-in called ``something.plugin``.
 
    .. code-block:: sh
 
-    $ pip install something
+    $ python3 -m pip install something
 
 #. Configure coverage.py to use the plug-in.  You do this by editing (or
    creating) your .coveragerc file, as described in :ref:`config`.  The