Fix normalization for multi-segment TLDs using Fastmail

[ryangsteele]

There are two changes in this MR:

1. Fix improper normalization for addresses with multi-segment TLDs whose domains use Fastmail MXs

The library currently assumes that if the LOCAL_PART_AS_HOSTNAME flag is set for a provider (as is the case with Fastmail), and the domain of the email address to be normalized is greater than 2 segments, that the last two segments are the domain. This logic works if the TLD is a single segment, like .com or .net, but fails with multi-segment TLDs like com.au and .co.uk.

For example, if the email address being normalized is foo+bar@baz.co.uk, and baz.co.uk's MX records indicate it's using Fastmail as a mail provider (e.g., in1-smtp.messagingengine.com.), then the email address is incorrectly normalized as baz@co.uk instead of foo@baz.co.uk.

To facilitate this change, I've made use of the tldextract library to correctly determine the TLD and avoid making assumptions based strictly upon the number of segments. Given that psl was recently archived, tldextract appeared to be the best available choice for this purpose.

2. Cache entries expired immediately if aiodns returned TTL of -1 for MX record lookup

aiodns was returning invalid (as per RFC 1035) TTL value of -1 for Gmail's MX records, causing cache entries to be considered immediately expired. This meant that on the second call to mx_records('gmail.com'), the cache entry was deleted and recreated instead of being reused, resetting the hit counter to 1. In such situations the tests would fail:

======================================================================
FAIL: test_cache (test_normalizer.NormalizerTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/ryans/git/email-normalize/tests/test_normalizer.py", line 23, in wrapper
    args[0].loop.run_until_complete(func[0](*args, **kwargs))
  File "/Users/ryans/.pyenv/versions/3.9.16/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/Users/ryans/git/email-normalize/tests/test_normalizer.py", line 80, in test_cache
    self.assertEqual(email_normalize.cache['gmail.com'].hits, 2)
AssertionError: 1 != 2

The change I implemented here is to filter out invalid TTL values (< 0) and fall back to failure_ttl when no valid TTL values are available.

[gmr]

Few thoughts:

1. I'm not keen on adding another 3rd party dependency. aiodns is there because it's needed to do asyncio DNS resolution.
2. I'm not sure what this is supposed to do or why it's a problem with this library

[ryangsteele]

Apologies, this MR absolutely needed some more context to describe the problem and proposed solution, as well as docs and a version bump.

I've since added all of these things; mind taking another gander and letting me know if the proposed change seems well-reasoned and appropriate, @gmr ?