chore: update docs with supported Python versions (#53)

* Update python versions

to match those in the tox.ini file

* update classifiers

to add recent python versions covered by tox.ini

README.rst

@@ -45,7 +45,7 @@ Or download the source and run the setup file (requires setuptools):
 
    $ python setup.py install
 
-Xenon is tested with all versions of Python from **2.7** to **3.6** as well as
+Xenon is tested with Python versions **2.7** and **3.6** to **3.9** as well as
 **PyPy**.
 
 Usage

setup.py

@@ -40,6 +40,10 @@
           'Programming Language :: Python :: 3.3',
           'Programming Language :: Python :: 3.4',
           'Programming Language :: Python :: 3.5',
+          'Programming Language :: Python :: 3.6',
+          'Programming Language :: Python :: 3.7',
+          'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
           'Topic :: Software Development',
           'Topic :: Software Development :: Libraries :: Python Modules',
           'Topic :: Software Development :: Quality Assurance',