master: Add support for Python 3.7

[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
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Add support for Python 3.7
• A small workaround is needed for Travis CI, see Add Python 3.7 option travis-ci/travis-ci#9815

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #615 into v4 will increase coverage by 0.39%.
The diff coverage is 74.86%.

@@            Coverage Diff             @@
##               v4     #615      +/-   ##
==========================================
+ Coverage   84.38%   84.77%   +0.39%     
==========================================
  Files          37       35       -2     
  Lines         967     1156     +189     
  Branches      108      172      +64     
==========================================
+ Hits          816      980     +164     
+ Misses         93       90       -3     
- Partials       58       86      +28

Impacted Files	Coverage Δ
sendgrid/helpers/mail/personalization.py	93.25% <100%> (+0.2%)	⬆️
sendgrid/version.py	100% <100%> (ø)	⬆️
sendgrid/helpers/stats/__init__.py	100% <100%> (ø)
sendgrid/helpers/endpoints/ip/unassigned.py	15% <15%> (ø)
sendgrid/helpers/mail/mail.py	93.71% <50%> (-6.29%)	⬇️
sendgrid/helpers/stats/stats.py	82.19% <82.19%> (ø)
sendgrid/helpers/mail/__init__.py	100% <0%> (ø)	⬆️
sendgrid/helpers/mail/validators.py	44.44% <0%> (ø)	⬆️
... and 13 more

---

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[thinkingserious]

Hello @hugovk,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[thinkingserious]

Can you please move this to the v4 branch? Thanks!

[hugovk]

Changing the base branch didn't work very smoothly! Closing this in favour of #637.