Add SEP-5 tests for Shamir, more documentation and examples

[tupui]

Follow up of #1012

(Note I will link the PR to the SEP-5 itself once I put it up.)

EDIT this is the update to the SEP stellar/stellar-protocol#1601

[overcat]

LGTM! Only a few details may be worth revising, and I have placed them in the comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.20%. Comparing base (4200224) to head (99ea57f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1015   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files         261      261           
  Lines       14066    14075    +9     
=======================================
+ Hits        13813    13822    +9     
  Misses        253      253           

Flag	Coverage Δ
unittests	98.20% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
stellar_sdk/keypair.py	98.98% <ø> (ø)
stellar_sdk/sep/mnemonic.py	100.00% <ø> (ø)
tests/test_keypair.py	100.00% <100.00%> (ø)

[tupui]

Adjusting for entropy instead of BIP-39

[overcat]

Thank you for this excellent PR! The code is clean and well-documented.
But I noticed that one test case failed; can you update it? Once the tests pass, we can merge it.

[tupui]

Done 😃

[tupui]

Oh wait there is something fishy.

[tupui]

ok I had to revert the to_entropy in the tests. See the comment in the other PR too. Depending on what you say there, I can adjust this with separate keys so we use entropy still.

[overcat]

LGTM