Updating function signatures and expected test outputs to match SG API v6

[petrklus]

Done and tested:

• Updated function signatures to support SendGrid v6
• Updated tests to match what was expected (omission of empty subject from Personalization)
• Tested sending with Django (tested adding attachments

What has not been tested:

• Templates using template_id (as I am not using any)
• Substitutions (again, not using any)

I think this should now require fairly small amount of work to get fully tested

[codecov-io]

Codecov Report

Merging #95 into master will increase coverage by 0.62%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   77.35%   77.98%   +0.62%     
==========================================
  Files           1        1              
  Lines         106      109       +3     
  Branches       30       30              
==========================================
+ Hits           82       85       +3     
  Misses         19       19              
  Partials        5        5              

Impacted Files	Coverage Δ
sgbackend/mail.py	77.98% <66.66%> (+0.62%)	⬆️

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[andriisoldatenko]

@petrklus please update codecov.

[petrklus]

thank you for the quick reply @andriisoldatenko - do you mean that I should write brand new tests for the file attachment dynamic, as well as substitutions?
Considering that the code was untested on master before, is that really necessary to implement new tests as a requirement for the PR to be merged? Curious!