Mailsuite

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- Fix `headers_string` value in parsed emails

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- Use `STARTTLS` in `IMAPClient` before login if the server supports it (PR 2)
- Properly handle cases where the IMAP separator is `None` (PR 4)
- Retry operations when disconnected from the server (PR 5)
- Add support for certificate authentication in IMAP (PR 7)
- Changes to email parsing
- Every header is now included as its own dictionary entry
- Remove redundant `headers` subdictionary
- Parse and normalize `Authentication-Results`, `Authentication-Results-Original`. amd `DKIM-Signature` headers
- `headers_str` now contains the full headers with indentations removed
- `raw_headers` includes the full headers without any modifications
- Add `base_domain` entry to `parse_email_address()` output
- Remove `six` requirement
- Set required `mail-parser` version to `>=1.14.0`
- Set required `dnspython` version to `>=2.0.0`

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- Remove copies of email headers from the root of the returned dictionary (still available under `headers`
- Store the HTML body in body in `body`, or the text body if an HTML body does not exist
- Store all body parts as a string in `raw_body`
- Only run `html2text` on HTML bodies
- Disable wrapping on markdown to make searching strings with YARA or other tools easier

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- Improve `get_filename_safe_string()`

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- Fix version numbering

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- Pin some dependency versions to avoid conflicts

- `six==1.13.0`
- `mail-parser==3.11.0`