Pyas2lib

* Use `signature_algo` attribute when detecting the signature algorithm
* Raise exception when unknown `digest_alg` is passed to the sign function
* Add proper support for handling binary messages
* Look for `Final-Recipient` if `Original-Recipient` is not present in the MDN
* Remove support for python 3.6
* Fix linting and change the linter to pylava

* Use correct format for setting dataclasses requirement for python 3.6

* Fix and update the SMIME capabilities in the Signed attributes of a signature
* Update the versions of crypto dependencies and related changes
* Use black and pylama as code formatter and linter
* Increase test coverage and add support for python 3.8

* Handle MDNNotfound correctly when parsing an mdn

* Handle exceptions raised when parsing signed attributes in a signature https://github.com/abhishek-ram/django-pyas2/issues/13
* Add more debug logs during build and parse
* Catch errors in MDN parsing and handle accordingly

* Use f-strings for string formatting.
* Use HTTP email policy for flattening email messages.
* Add proper support for other encryption algos.
* Use dataclasses for organization and partner.
* Remove support for python 3.5.
* Add utility function for extracting info from certificates.