Dump bridges to file, despite lack of BridgeHistory

We only dumped the bridges assigned to buckets if we had
bridgehistory for them. This is not so useful if we disable
the stability code which tracks this. We now dump all bridges
that are assigned to a distributor to file when the stability
code is disabled.
Add a test for Buckets

The others will require mocking a DB connection.
Merge branch 'fix/9874-https' into develop
Add CHANGELOG entry for #9874.
Merge branch 'fix/9874-email' into develop
Clarify comments in .travis.requirements.txt.
Merge remote-tracking branch 'tpo-sysrqb/bug12781-dumpbuckets' into develop
Fix test_https tests to handle multiple bridge lines.

See https://travis-ci.org/isislovecruft/bridgedb/jobs/33035309#L1781 for an
example of how this test was failing.
Fix missing `twisted.trial.unittest` import in test_Bucket.py.
Fix missing `io.StringIO` import in test_Bucket.py.
Fix typo in inline code comment in bridgedb.Bucket module.
Fix test_Bucket.BucketDataTests to test `needed` attribute.
Fix old-style classes in bridgedb.Bucket.
Sphinxify all docstrings in bridgedb.Bucket.
Fix another typo in test_Bucket.py.
Constify the unrealistic maximum number of bridges per bucket

We will never, ever have 1 million bridges, but because of this we
will also never be able to satisfy the requirement of giving a bucket
one million bridges. So, when we try distributing the bridges evenly
between buckets, the pseudo-distributors that are defined as needing
'*' bridges will receive as many as are available to be allocated to
them.
Add missing unittest import
Line wasn't indented in f5fe05ce
Merge remote-tracking branch 'tpo-sysrqb/bug12781-dumpbuckets_constification_2' into develop

Conflicts:
	lib/bridgedb/Bucket.py
	lib/bridgedb/test/test_Bucket.py
Fix yet another type error in test_Bucket.py.