Ambiguity with BSD license trove classifier

[thatch]

PEP 639 will make this a little better, but once documentation exists we should at least document that looping through the licenses looking for a trove classifier match is ambiguous in some very common cases.

[spoorcc]

#6 adds a test that checks all trove classifiers are covered. If any new classifiers are introduced the test will start failing indicating which should be added.