Add tox.ini to enable running tests using tox

Add `tox.ini` file that will enable us to use
[tox](https://tox.readthedocs.io/) as a frontend for running tests.
Since we share the config via `setup.cfg` and requirements via
requirements files, commands like `python3 setup.py test` will continue
to work.

.gitignore

@@ -17,6 +17,7 @@ tmp
 .cache
 .pytest_cache/
 *.bst/
+.tox
 
 # Pycache, in case buildstream is ran directly from within the source
 # tree

MANIFEST.in

@@ -24,6 +24,7 @@ recursive-include doc/sessions *.run
 # Tests
 recursive-include tests *
 include conftest.py
+include tox.ini
 include .coveragerc
 include .pylintrc
 

tox.ini

+[tox]
+envlist = py35,py36,py37
+skip_missing_interpreters = true
+
+[testenv]
+commands = pytest {posargs}
+deps =
+    -rrequirements.txt
+    -rdev-requirements.txt
+    -rplugin-requirements.txt
+passenv =
+    GI_TYPELIB_PATH
+    INTEGRATION_CACHE