chore(deps): migrate to RxPY v4; set minimal Python version to 3.7

[bednar]

Proposed Changes

1. Migrate to RxPY v4

   https://rxpy.readthedocs.io/en/latest/migration.html

2. Set minimal Python version to 3.7 (required by RxPy v4)2.
   

   https://peps.python.org/pep-0494/

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #472 (f4222f9) into master (f02716a) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
- Coverage   89.50%   89.38%   -0.13%     
==========================================
  Files          38       38              
  Lines        3306     3306              
==========================================
- Hits         2959     2955       -4     
- Misses        347      351       +4     

Impacted Files	Coverage Δ
influxdb_client/client/write_api.py	99.01% <100.00%> (ø)
influxdb_client/client/influxdb_client_async.py	87.93% <0.00%> (-3.45%)	⬇️
influxdb_client/client/_base.py	97.34% <0.00%> (-0.59%)	⬇️

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[rhajek]

LGTM