Make use of Python's email package for EmaiMessage types

[Christovis]

This PR addresses #434.

It inclues:

• changing ListservMessage to ListservMessageParser and using email.message.EmailMessage together with mailbox.mboxMessage to represent a ListservMessage
• editing listserv tests to according to the changes above
• isort and black formatting have become incompatible so that we continue to use only black code formatting
• edit the priority in which message payloads are stored in the following way: 1) text/plain 2) text/html, otherwise return None
• some messages seems to cause error executing mailbox.mbox() .add(), they are captured on exception and recorded through the logger

[Christovis]

As all test have been adapted to the changes made and the code shows to work smoothly even appr. 12h into scraping 3GPP from the web, I suggest we merge it.