converted README to reStructuredText and version as plain text file

[StrikerRUS]

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
• [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:

• simplify processing of __version__ attribute by retrieving it from the plain text file, not by executing the .py module.
• convert README.md to README.rst (appearance can be checked in this branch https://github.com/StrikerRUS/sendgrid-python/blob/pypi/README.rst), so register.py is not needed anymore.
• add Python versions badge.
  
• add PyPI version badge.
  

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #634 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
- Coverage   84.77%   84.74%   -0.03%     
==========================================
  Files          35       34       -1     
  Lines        1156     1154       -2     
  Branches      172      171       -1     
==========================================
- Hits          980      978       -2     
  Misses         90       90              
  Partials       86       86

Impacted Files	Coverage Δ
sendgrid/sendgrid.py	92% <100%> (ø)	⬆️

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[thinkingserious]

Hello @StrikerRUS,

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]

Hi @StrikerRUS!

Could you please make this PR against the v4 branch? Thanks!

With Best Regards,

Elmer

[StrikerRUS]

@thinkingserious I'm so sorry, but I've failed to rebase the branch and moved all changes from this PR to #663.