master: Use raw-string notation for regex to avoid invalid escape sequence

[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.
• [n/a] I have added tests that prove my fix is effective or that my feature works
• [n/a] 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:

• Use raw-string notation for regex to avoid invalid escape sequence

From the re docs:

Regular expressions use the backslash character ('\') to indicate special forms or to allow special characters to be used without invoking their special meaning. This collides with Python’s usage of the same character for the same purpose in string literals; for example, to match a literal backslash, one might have to write '\\\\' as the pattern string, because the regular expression must be \\, and each backslash must be expressed as \\ inside a regular Python string literal.

The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So r"\n" is a two-character string containing '\' and 'n', while "\n" is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

Before

$ pycodestyle | grep W605
./sendgrid/helpers/mail/validators.py:27:16: W605 invalid escape sequence '\.'
./sendgrid/helpers/mail/validators.py:27:30: W605 invalid escape sequence '\.'
$

After

$ pycodestyle | grep W605

Once merged, the exception from #654 should be removed:

https://github.com/sendgrid/sendgrid-python/pull/654/files#diff-354f30a63fb0907d4ad57269548329e3R35

[codecov]

Codecov Report

Merging #698 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
- Coverage   85.04%   84.77%   -0.28%     
==========================================
  Files          35       35              
  Lines        1157     1156       -1     
  Branches      172      172              
==========================================
- Hits          984      980       -4     
  Misses         90       90              
- Partials       83       86       +3

Impacted Files	Coverage Δ
sendgrid/helpers/mail/validators.py	44.44% <100%> (ø)	⬆️
sendgrid/helpers/mail/spam_check.py	88.46% <0%> (-7.7%)	⬇️
sendgrid/helpers/inbound/config.py	93.93% <0%> (-3.12%)	⬇️
sendgrid/helpers/mail/exceptions.py	66.66% <0%> (ø)	⬆️
sendgrid/helpers/inbound/send.py	93.75% <0%> (ø)	⬆️
sendgrid/helpers/mail/content.py	84.61% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 90cfbf2...37f0486. Read the comment docs.

[misterdorm]

Would you please rebase this against the v4 branch? Otherwise, LGTM. Thanks!

[misterdorm]

Rebase to v4.

[hugovk]

Please see #727 as a replacement on the v4 branch.