Simplify Python process launch in light of commit aac194b.

Ever since commit aac194b there have been intermittent "broken pipe"
errors only on Travis CI. My hunch is that these are caused by the 3
second timeout -- Travis CI can sometimes be a slow environment -- and
attempting to read from the pipe input port too soon.

We could increase the timeout. But I no longer see the point of having
any timeout -- including in a "real" non-Travis situation.

More fundamentally, also we no longer need a separate thread to launch
Pygments. Originally that allowed Python startup to complete earlier in
the Frog startup. But commit aac194b requires waiting in order to pass
Python some command-line flags that are read from frogrc. So the extra
thread is a needless complication, these days.

TL;DR: Start the Python process and try to read its "ready" line. Wait
as long as necessary. If the process dies, we'll read EOF. All's well
either way.