Add tests to source distribution

Author: Cito

Diff for: MANIFEST.in

@@ -1,4 +1,13 @@
-global-exclude tests/*
-recursive-exclude tests *
-recursive-exclude tests_py35 *
+include MANIFEST.in
+
 include LICENSE
+include README.md
+
+include .flake8
+
+include tox.ini
+
+graft src/graphql_relay
+graft tests
+
+global-exclude *.py[co] __pycache__

Diff for: tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,34,35,36,37,py,py3}, flake8
+envlist = py{27,34,35,36,37,py,py3}, flake8, manifest
 
 [testenv:flake8]
 basepython = python3.7
@@ -13,6 +13,12 @@ whitelist_externals=*
 [testenv:pypy3]
 whitelist_externals=*
 
+[testenv:manifest]
+basepython = python3.7
+deps = check-manifest>=0.40,<1
+commands =
+    check-manifest -v
+
 [testenv]
 commands=
     python setup.py test -a "{posargs}"