Support core metadata versions 2.3 and 2.4

[andriygm]

closes #34

[jmwoliver]

@andriygm Thanks again for this contribution as well! It looks great, but do you know of a package that could be used to exercise this metadata in a test? If there is a public package we know uses that field, we could add it to the repo list here:

python-distribution-parser/parse_test.go

Line 39 in 1b78f81

var repositories = []Repository{

Then the tests would verify that the fields from 2.3 and 2.4 behave the same as the latest twine version.