They're all redundant on Python 3

and fix it

as Tahoe now wants, so it can respect available space when allocating more for
a lease

The version of versioneer has an issue that the tarball that is generated on
github changes, when the branches and/or tags refering to a commit change. This
causes issues for gridsync, which pins the hashes of those tarballs.

In addition to that, versioneer doesn't provide a meaningful revision for
untagged commits, when run from a github tarball.

Given that we only ever consume a github tarball as a github archive (except in
local development, where the version number is less important), this replaces
versioneer with some hand-rolled code specialized to that use case.

The later function takes a count of passes to generated, and passes
them to `make_passes`. This also changes `make_passes` to return `Pass`es,
which is what `get_passes` wants to receive.

also rename and clarify the read_bucket `size` parameter

The test demonstrates this now by depending on part of the successful result
from the underlying storage server object.

It's more specific and less powerful, therefore more expressive and safe.

Co-authored-by: Tom Prince <tom.prince@private.storage>