Added support for Python 3.7 and 3.8 (#55)

This change introduces the Python versions 3.7 and 3.8 to the continuous
integration.

Author: mristin

.travis.yml

@@ -2,6 +2,8 @@ language: python
 python:
   - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
 install:
   - pip3 install -e .[dev]
   - pip3 install coveralls

setup.py

@@ -36,6 +36,8 @@
         'License :: OSI Approved :: MIT License',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8'
     ],
     license='License :: OSI Approved :: MIT License',
     keywords='object mapping json serialization deserialization graph',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py36
+envlist = py35,py36,py37,py38
 
 [testenv]
 deps =  .[dev]