add .gitlab-ci.yml that uses tox

.gitlab-ci.yml

+image: python
+
+before_script:
+  - pip install tox
+
+flake8:
+  script: tox -e flake8
+
+py3:
+  script: tox -e py3
+
+check-manifest:
+  script: tox -e check-manifest
+
+black:
+  script: tox -e black
+
+mypy:
+  script: tox -e mypy

MANIFEST.in

@@ -16,3 +16,4 @@ include __pkginfo__.py
 prune debian
 prune docs/_build
 prune .hgrc
+prune .gitlab-ci.yml