Patch RF to handle root with single descendant

[colganwi]

@isabellechan089 your _robinson_foulds_bitset implementation raises an error when the root has a single descendant because leaves1 = sorted([n for n in tree1 if tree1.degree[n] == 1]) will incorrectly include it in the leaf set. I've patched it here but let's watch out for this failure mode in the future and be sure to include more tree topologies in the tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.92%. Comparing base (071560e) to head (f740031).
⚠️ Report is 2 commits behind head on 3.0.0.

Additional details and impacted files

@@           Coverage Diff           @@
##            3.0.0     #282   +/-   ##
=======================================
  Coverage   86.91%   86.92%           
=======================================
  Files          76       76           
  Lines        5922     5918    -4     
=======================================
- Hits         5147     5144    -3     
+ Misses        775      774    -1     

Files with missing lines	Coverage Δ
src/cassiopeia/critique/compare.py	96.70% <100.00%> (+0.91%)	⬆️

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