We recently updated the `Pypi` strategy to use the PyPI JSON API and
the default strategy behavior no longer relies on a regex, so the initial implementation didn't include regex handling. This restores
support for a `livecheck` block regex by updating the `DEFAULT_BLOCK`
logic to handle an optional regex. This allows us to use a regex to
omit parts of the `info.version` value without having to duplicate
the default block logic in a `strategy` block only to use a regex.
This isn't currently necessary for any existing formulae using the
`Pypi` strategy but we have a few that needed a custom regex with
the previous strategy approach, so they may need this functionality
in the future. Besides that, restoring regex support to `Pypi`
ensures that `livecheck`/`strategy` blocks work in a fairly
consistent manner across strategies.
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