converted README to reStructuredText and version as plain text file

[StrikerRUS]

This is the copy of #634 PR, but against v4 branch

Important: content of README was taken from master branch, since there are more recent fixes and tracking of all of them will be lost after converting the format of the file, with applied patch for recent Python version update in v4 branch.

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_v4/README.rst), so register.py is not needed anymore.
• add Python versions badge.
  
• add PyPI version badge.
  

[codecov]

Codecov Report

Merging #663 into v4 will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##               v4     #663      +/-   ##
==========================================
- Coverage   84.38%   84.35%   -0.04%     
==========================================
  Files          37       36       -1     
  Lines         967      965       -2     
  Branches      108      107       -1     
==========================================
- Hits          816      814       -2     
  Misses         93       93              
  Partials       58       58

Impacted Files	Coverage Δ
sendgrid/sendgrid.py	85.71% <100%> (ø)	⬆️

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[meahow]

Why change the README format?

[StrikerRUS]

@meahow

so register.py is not needed anymore.

[thinkingserious]

@meahow, .rst is the format PyPi supports. Supporting this natively simplifies things. As @StrikerRUS said, now there is no need for converting the .md to .rst with register.py.

Do you see any issue with this approach? Thanks!

[meahow]

@thinkingserious The only issue I have with this is that MD syntax is a bit simpler/clearer.

FYI pypi now supports MD but:

• you need python 3.x (probably only for creating a package)
• you need to upload the package with twine

More info here

[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