(to reflect the type that is now being tested)

  (The comment for this mixin class says
    # additional tests that only work for
    # stringlike objects, i.e. str, UserString
   but in the absence of `unicode` the `CommonTest` class is also only
   used for these two types now, so no need for the distinction.)

  communicate its purpose, and to disambiguate it from the more general
  `BaseTest` class, which covers behaviours shared with the `bytes`
  and `bytesarray` types.

  (Seems to date back to the first introduction of this code, in 2003!)

  non-str types. Back in 2007 there was a change to allow str.join() to
  convert non-str values automatically, but this is no longer the case.

  raising TypeError, so BadSeq1 and the associated check seem to be
  redundant. This is the existing check, in test_join():
      self.assertRaises(TypeError, '.'.join, ['a', 'b', 3])
  (Unless there is some specific difference we're testing by supplying a
   custom sequence, BadSeq1, rather than a plain list?)
- Define BadSeq2 closer to its only usage, and give it a more meaningful
  name. (Previously this type was referenced from multiple locations but
  this is no longer the case.)