chore(docs): Update README.md for python 3.7

[chiral-carbon]

Fixes #6520

Short description of what this resolves:

Updated README.md to reflect that open-event-server now runs on python 3.7.

Changes proposed in this pull request:

• Documentation fix.

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[chiral-carbon]

@prateekj117 @iamareebjamal please review.

[iamareebjamal]

Please search elsewhere as well, if we have written python < 3.7

[codecov]

Codecov Report

Merging #6535 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #6535   +/-   ##
============================================
  Coverage        65.11%   65.11%           
============================================
  Files              295      295           
  Lines            15284    15284           
============================================
  Hits              9952     9952           
  Misses            5332     5332

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[chiral-carbon]

@iamareebjamal I can't find <python 3.7 mentioned anywhere else in README.md or the installation docs. But should I change python 3 in the requirements in the installation md files as mentioned here to python 3.7?

[chiral-carbon]

@iamareebjamal please check.