Improve code quality

[andriisoldatenko]

Summary

According to CONTRIBUTING code should pass all pep8 and flake8 errors

On master branch we can see

(python-sedgrid) ~/sendgrid-python$ flake8 . | wc -l
1201
OR
pep8 . | wc -l
1033

The goal of this PR to fix all errors.

After this PR merged:

flake8 test/ | wc -l 
test/test_mail.py:212:80: E501 line too long (3359 > 79 characters)

Checks on sendgrid/ module:

flake8 sendgrid/ wc -l

Unit tests:

----------------------------------------------------------------------
Ran 238 tests in 32.903s

OK

Install checking tools:

pip install flake8 pep8 isort

[andriisoldatenko]

@thinkingserious Please review.
cc @vananabu

[thinkingserious]

Awesome @andriisoldatenko!

Could you please sign our CLA so I can review and merge? Thanks!

[andriisoldatenko]

@thinkingserious done.

[thinkingserious]

Hi @andriisoldatenko,

Could you please fix the conflicts in test/test_sendgrid.py? Thanks!

[andriisoldatenko]

@thinkingserious done.

[andriisoldatenko]

@thinkingserious Can you please review again. Thanks

[SendGridDX]

Hi @andriisoldatenko,

Thanks again for the PR, please take a moment to fill out this form so that we may send you some swag :)