Fix raw-HTML pattern in README parsing

[EpicWink]

Fixes the issue introduced by #1311 where all of the README's contents are not in the published package's long-description.

Also explicitly specify README as RST.

Explanation:

• Removing s flag - so dots . don't match newlines. This made the capturing group too greedy
• Making the group's quantifier * not greedy, so the matched text ends as soon as the rest of the pattern can be satisfied
• Specify end delimiter \n\n - this ends all RST blocks, so we match (and remove) that delimiter
• Specifying RST as the content-type is good practice, even if PyPI assumes RST

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.10%. Comparing base (ff3dcb9) to head (51151c5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1359   +/-   ##
=======================================
  Coverage   97.10%   97.10%           
=======================================
  Files         236      236           
  Lines        3008     3008           
=======================================
  Hits         2921     2921           
  Misses         87       87           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[EpicWink]

Failing build is due to mismatch between README and changelog (aka HISTORY) regarding section title underlines. There's two options to fix this:

1. Add a new top-level section title to the README, with equal-sign = underlining ¹. This would demote the heading levels of all current titles in the README
2. Change the README's section titles' underlines to match HISTORY (ie top-level from hyphen - to equal-sign =, second-level from carat ^ to hyphen -)

Footnotes

1. You could convert the new bold text in Specify README header content as RST #1360 to this heading ↩

[AdrianAtZyte]

I am fine with either.

[EpicWink]

I chose the second option, to have the formatted README look the same