feat: SEP-10, adds a 5 minute grace period to read_challenge_transaction min_time constraint

[overcat]

#524

[codecov]

Codecov Report

Merging #526 (93d46ed) into v4 (8d40fbf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v4     #526   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files         129      129           
  Lines        8987     9019   +32     
=======================================
+ Hits         8901     8933   +32     
  Misses         86       86           

Flag	Coverage Δ
unittests	99.04% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
stellar_sdk/sep/stellar_web_authentication.py	94.76% <100.00%> (+0.02%)	⬆️
tests/sep/test_stellar_web_authentication.py	100.00% <100.00%> (ø)