adjust readme and setup description to reflect version support change

Author: rneher

README.md

@@ -47,7 +47,7 @@ Have a look at our repository with [example data](https://github.com/neherlab/tr
 
 ### Installation and prerequisites
 
-TreeTime is compatible with Python 2.7 upwards and is tested on 2.7, 3.5, and 3.6.  It depends on several Python libraries:
+TreeTime is compatible with Python 3.5 upwards and is tested on 3.5, 3.6, 3.7, and 3.8.  It depends on several Python libraries:
 
 * numpy, scipy, pandas: for all kind of mathematical operations as matrix
   operations, numerical integration, interpolation, minimization, etc.

setup.py

@@ -39,9 +39,10 @@ def get_version():
             "Development Status :: 3 - Alpha",
             "Topic :: Scientific/Engineering :: Bio-Informatics",
             "License :: OSI Approved :: MIT License",
-            "Programming Language :: Python :: 2.7",
             "Programming Language :: Python :: 3.5",
-            "Programming Language :: Python :: 3.6"
+            "Programming Language :: Python :: 3.6",
+            "Programming Language :: Python :: 3.7",
+            "Programming Language :: Python :: 3.8"
             ],
         scripts=['bin/treetime']
     )