Checkdmarc

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- Make DMARC retorting URI error messages more clear (PR 104)

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- Fix compatibility with Python 3.8

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- `SPFRecordNotFound` exception now includes a `domain` argument (PR 103)
- The DMARC missing authorization error message now includes the full expected DNS record
- Lots of code cleanup
- Added missing docstrings

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- `get_base_domain()` will return the input string instead of `None` if it can't parse a domain
- Always use the base domain when testing DNSSEC

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- Fix DNSSEC test
- Do not treat `include` mechanisms with macros as domains (Close issue 81)
- Add `DMARCRecordStartsWithWhitespace` exception (PR 97)
- Properly parse DMARC and BIMI records for domains that do not have an identified base domain (PR 98)
- Add `ignore_unrelated_records` argument to `query_dmarc_record()` (Slight modification of PR 99 - Close issue 91)
- Mark syntax error positions (Slight modification of PR 100)

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- Break up code into smaller methods (PR 93)