setup: bump pyvcd to ~=0.2.

setup: bump pyvcd to ~=0.2.

We don't use any of the deprecated functionality, and the added
requirement of Python 3.6+ matches ours.

Having a requirement for pyvcd ~=0.1.4 was actually the cause of
the error I tried to fix in commit 6e1145e2. It had nothing to do
with Jinja2 (though it is definitely still good to have the tighter
requirement on Jinja2); the cause of the error was that pip would
install Jinja2, schedule installing markupsafe, install pyvcd 0.2.0
and then choke on the pyvcd 0.1.4 requirement, which would prevent
it from installing markupsafe &c. Why it does that is beyond me.