faster reading of h5ad file (~18X faster)

[kaushalprasadhial]

This PR is for faster reading of h5ad file (~18X faster).

	Time
Original	74.79
Updated	4.09
Speedup	18.27

Testing was done in AWS m7i.24xlarge

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 21.95122% with 64 lines in your changes missing coverage. Please review.

Project coverage is 58.65%. Comparing base (9d3c340) to head (3d2f06f).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	21.95%	64 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3365       +/-   ##
===========================================
- Coverage   77.26%   58.65%   -18.62%     
===========================================
  Files         111      111               
  Lines       12630    12946      +316     
===========================================
- Hits         9759     7593     -2166     
- Misses       2871     5353     +2482     

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	68.64% <21.95%> (-9.11%)	⬇️

... and 53 files with indirect coverage changes

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