Replace sortedcontainers with builtin collections

[pavoljuhas]

Eliminate unnecessary dependency on the sortedcontainers package.
Used in only one module where SortedSet could be ordered just once
after collection and where SortedDict could be also sorted later.

The median timeit timing of the affected get_initial_mapping calls
from unit test improved by about 20%.

[pavoljuhas]

To check how performance changes with SortedSet replacement, here are timeit durations from initialization_test.py collected using

pytest -s --randomly-seed=99 --randomly-dont-reorganize --pyargs \
    cirq.contrib.routing.initialization_test

and plotted as a histogram of durations before and after this PR:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.63%. Comparing base (3175c9d) to head (db7fc69).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8020      +/-   ##
==========================================
- Coverage   99.63%   99.63%   -0.01%     
==========================================
  Files        1110     1110              
  Lines       99755    99755              
==========================================
- Hits        99394    99392       -2     
- Misses        361      363       +2     

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[mhucka]

Nice.