Add release job to tox

This also changes the build job to clear out existing entries before a
new build is invoked.

Author: pganssle

tox.ini

@@ -48,5 +48,21 @@ deps =
     pep517
     twine
 commands =
+    python -c "from pathlib import Path; \
+               [x.unlink(missing_ok=True) for x in Path('{toxinidir}/dist').glob('*')]"
     python -m pep517.build -s -b {toxinidir} -o {toxinidir}/dist
     twine check {toxinidir}/dist/*
+
+[testenv:release]
+description = Make a release; must be called after "build"
+skip_install = True
+deps =
+    twine
+depends =
+    build
+passenv =
+    TWINE_*
+commands =
+    twine check {toxinidir}/dist/*
+    twine upload {toxinidir}/dist/* \
+                 {posargs:-r testpypi --non-interactive}