Ensure Matcher.regex raises when passed non-string

[rohanpm]

Matcher.regex is documented as accepting a string, but previously
if you passed an already-compiled regex, that would also be
accepted by the validator. This happened because re.compile simply
returns already-compiled regexes without an error. It would
ultimately lead to an error when executing a search, because a
compiled regex is not serializable to JSON.

Fix it to more strictly typecheck the inputs when creating the
matcher, so we fail early and with a meaningful message.

[rohanpm]

CI is failing as the reverse-dependency test is showing that pubtools-pulp tests would fail with this change. That's expected until release-engineering/pubtools-pulp#15 is submitted. The issue is that pubtools-pulp currently relies on this missing validation for its own autotests.

So, this is fine for review now, but release-engineering/pubtools-pulp#15 must be submitted and then this should be retested before merging.