tox += lint

Add minimal linting step that verifies manifest and built sdist/wheel.
Suggested by @bertjwregeer on https://github.com/Pylons/zodburi/pull/29#issuecomment-836915440.

tox.ini

 [tox]
 envlist =
+    lint
     {py27,py35,py36,pypy,pypy3}-{zodb4,zodb5},{py37,py38}-zodb5,cover,docs
 
 [testenv]
@@ -37,3 +38,17 @@ commands =
 deps =
     Sphinx
     pylons-sphinx-themes
+
+
+[testenv:lint]
+skip_install = true
+commands =
+    check-manifest
+    # build sdist/wheel
+    python setup.py sdist
+    python setup.py bdist_wheel
+    twine check dist/*
+deps =
+    check-manifest
+    readme_renderer
+    twine