DOC: Specify py2k-compatible version in py3k statement

The `python>=3.6` requirement at the top of the file dates to early 2020, before v0.1 of MONETIO.  Before this, MONET and MONETIO were one package.  It looks like the last commit in MONET 2.1.5 merges in the work to split MONET and MONETIO, so someone wanting MONETIO functionality from MONET would need a MONET version prior to that.

docs/installing.rst

@@ -50,10 +50,10 @@ or you can manually download it from GitHub and install it using the setup.py::
 
 .. note::
    MONET plans to drop support for python 2.7 sometime in 2019. This
-   means that new releases of xarray published after this date will only be
-   installable on python 3+ environments, but older versions of xarray will
-   always be available to python 2.7 users. For more information see the
-   following references:
+   means that new releases of MONET and MONETIO published after this date will
+   only be installable on python 3+ environments, but older versions of MONET 
+   (<2.1.5) will always be available to python 2.7 users. For more information
+   see the following references:
 
       - `Python 3 Statement <https://python3statement.github.io/>`__
       - `Tips on porting to Python 3 <https://docs.python.org/3/howto/pyporting.html>`__