The .appveyor.yml tells the tox.ini to install pypiwin32 directly,
rather than through a "twisted[windows]" extra. I wasn't able to get the
"extra" syntax to work properly under appyeyor, even though it worked ok
for me locally.
We use an extra "meta" tox environment to run "wormhole --version". This
confirms that versioneer is getting the right value, since we're now
running it from an installed sdist, rather than a checked-out git source
tree.
