Fix markup used in the documentation of sys.prefix and sys.exec_prefix.

- Using the file role with {placeholders} is IMO clearer than fake
Python code.

- The fact that sys.version[:3] gives '2.7' is a CPython detail and
should not be advertised (see #9442), even if some stdlib modules
currently rely on that detail.

Doc/library/sys.rst

@@ -207,10 +207,10 @@ always available.
    Python files are installed; by default, this is also ``'/usr/local'``.  This can
    be set at build time with the ``--exec-prefix`` argument to the
    :program:`configure` script.  Specifically, all configuration files (e.g. the
-   :file:`pyconfig.h` header file) are installed in the directory ``exec_prefix +
-   '/lib/pythonversion/config'``, and shared library modules are installed in
-   ``exec_prefix + '/lib/pythonversion/lib-dynload'``, where *version* is equal to
-   ``version[:3]``.
+   :file:`pyconfig.h` header file) are installed in the directory
+   :file:`{exec_prefix}/lib/python{X.Y}/config', and shared library modules are
+   installed in :file:`{exec_prefix}/lib/python{X.Y}/lib-dynload`, where *X.Y*
+   is the version number of Python, for example ``2.7``.
 
 
 .. data:: executable
@@ -766,10 +766,10 @@ always available.
    independent Python files are installed; by default, this is the string
    ``'/usr/local'``.  This can be set at build time with the ``--prefix``
    argument to the :program:`configure` script.  The main collection of Python
-   library modules is installed in the directory ``prefix + '/lib/pythonversion'``
+   library modules is installed in the directory :file:`{prefix}/lib/python{X.Y}``
    while the platform independent header files (all except :file:`pyconfig.h`) are
-   stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
-   ``version[:3]``.
+   stored in :file:`{prefix}/include/python{X.Y}``, where *X.Y* is the version
+   number of Python, for example ``2.7``.
 
 
 .. data:: ps1