PEP8 Fixes and String formatting enhancement

[vkmrishad]

Fixes #673

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:

• PEP 8 Fixes.
• String formatting enhancement.

If you have questions, please send an email to SendGrid, or file a GitHub Issue in this repository.

[SendGridDX]

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[codecov]

Codecov Report

Merging #673 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   84.77%   84.78%   +0.01%     
==========================================
  Files          35       35              
  Lines        1156     1157       +1     
  Branches      172      172              
==========================================
+ Hits          980      981       +1     
  Misses         90       90              
  Partials       86       86

Impacted Files	Coverage Δ
sendgrid/helpers/inbound/send.py	93.75% <ø> (ø)	⬆️
sendgrid/helpers/mail/validators.py	44.44% <ø> (ø)	⬆️
sendgrid/helpers/mail/content.py	84.61% <ø> (ø)	⬆️
sendgrid/helpers/mail/exceptions.py	66.66% <ø> (ø)	⬆️
sendgrid/helpers/inbound/config.py	94.11% <100%> (+0.17%)	⬆️

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[misterdorm]

Looks good, thanks for this. You'll need to agree to the CLA before this can be merge.

[vkmrishad]

@misterdorm, I have already signed CLA. But showing "not signed yet" banner.

Is there any need for approval from SendGrid?