remove references to python 2

Author: zingale

README.md

@@ -38,26 +38,20 @@ https://pyro2.readthedocs.io/
  * [Working with data](#working-with-data)
  * [Understanding the algorithms](#understanding-the-algorithms)
  * [Regression and unit testing](#regression-and-unit-testing)
- * [python 2.7](#python-27)
  * [Acknowledgements](#acknowledgements)
  * [Getting help](#getting-help)
 
 
 ## Getting started
 
-  - By default, we assume python 3.4 or later.  Instructions to
-    run with python 2.7 are given below, but it is recommended you
-	switch to python 3.x
+  - By default, we assume python 3.6 or later.
 
   - There are a few steps to take to get things running. You need to
     make sure you have `numpy`, `numba`, `matplotlib`, and `h5py`
     installed. On a Fedora system, this can be accomplished by doing:
 
        `dnf install python3-numpy python3-numba python3-matplotlib python3-matplotlib-tk python3-h5py`
 
-    (note, for older Fedora releases, replace `dnf` with `yum`.  For
-	python 2.x, leave off the `3` in the package names.)
-
   - Not all matplotlib backends allow for the interactive plotting as
     pyro is run. One that does is the TkAgg backend. This can be made
     the default by creating a file `~/.matplotlib/matplotlibrc` with
@@ -257,22 +251,6 @@ with their data.
   http://bender.astro.sunysb.edu/hydro_by_example/download/_stage/pyro2/tests.out
 
 
-## python 2.7
-
-  To run with python 2.7, you need to build the compiled code as:
-
-  ```
-  PYTHON=python2 ./mk.sh
-  ```
-
-  and then run explicitly giving the name of the python interpreter
-  on the commandline as, for example:
-
-  ```
-  python2 ./pyro.py compressible sedov inputs.sedov
-  ```
-
-
 ## Acknowledgements
 
   If you use pyro in a class or workshop, please e-mail us to let us know