master: Drop support for EOL Python 2.6 and 3.0-3.3

[hugovk]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• [n/a] I have added in line documentation to the code I modified

Short description of what this PR does:

• Drop support for EOL Python 2.6 and 3.0-3.3

Python 2.6 and 3.0-3.3 are EOL and no longer receiving security updates (or any updates) from the core Python team.

Version	Release date	Supported until
2.6	2008-10-01	2013-10-29
3.0	2008-12-03	2009-06-27
3.1	2009-06-27	2012-04-09
3.2	2011-02-20	2016-02-27
3.3	2012-09-29	2017-09-29

They're also little used.

Here's the pip installs for sendgrid from PyPI for September 2018:

python_version	percent	download_count
2.7	43.21%	86,224
3.6	36.21%	72,262
3.4	9.08%	18,127
3.5	8.22%	16,410
3.7	3.03%	6,044
2.6	0.19%	374
3.3	0.06%	114
None	0.00%	1
Total		199,556

Source: pypinfo --start-date 2018-09-01 --end-date 2018-09-30 --percent --markdown sendgrid pyversion

I see #352 (comment) notes support for 2.6 is planned to be dropped in the next SDK version, and I don't think 3.0-3.2 were supported anyway.

This PR also upgrades code to use newer Python syntax, removes some old compatibility code, and fixes some flake8 and inspection warnings.

[codecov]

Codecov Report

Merging #617 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #617   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files          35       35           
  Lines        1156     1156           
  Branches      172      172           
=======================================
  Hits          980      980           
  Misses         90       90           
  Partials       86       86

Impacted Files	Coverage Δ
sendgrid/helpers/mail/validators.py	44.44% <ø> (ø)	⬆️
sendgrid/helpers/inbound/send.py	93.75% <ø> (ø)	⬆️
sendgrid/helpers/endpoints/ip/unassigned.py	15% <ø> (ø)	⬆️
sendgrid/helpers/mail/content.py	84.61% <ø> (ø)	⬆️
sendgrid/sendgrid.py	92% <100%> (ø)	⬆️
sendgrid/helpers/mail/exceptions.py	66.66% <100%> (ø)	⬆️
sendgrid/helpers/inbound/app.py	78.26% <100%> (ø)	⬆️

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[thinkingserious]

Hello @hugovk,

Could you please move these changes to this branch? Thanks!

With Best Regards,

Elmer

[hugovk]

Please see PR #626.