This makes the logic in the parser a little bit more sophisticated, but
we also start handling the yank reason. This fixes the issue where the
`data-yank` presence but no value would be interpreted as a yanked
package. With this it should start working.

This implementation assumes that we have HTML escaped sequences as tag
values. It also unescapes them when returning the strings.

The posibilities that it gives us are:
- Use the `data-requires-python` to potentially discard any Python
  packages that are unsupported in the `select_whl` function.

Work towards bazel-contrib#260.
Work towards bazel-contrib#2731.