We only have one version of tahoe-lafs here right now anyway, so the
identifier isn't ambiguous without a version number, and there are probably
more changes required to really support more than one version at a time.

Also limit build concurrency to 8.  A lot of the tests are I/O bound so they
are bottlenecked and extra cores don't help.

It was consistent so it worked fine but it was misleading since it points at
21.05, not the non-existent 20.15.

It turns our that `pyflakes` with no arguments expects input
on stdin. In CI, this is likely connected to `/dev/null` so it
passes.

Also fix the errors that pyflakes is reporting.

broken now because of dependency problems