Fix TypeError when a keypath sets a string subkey through a list value

[santhreal]

Setting a keypath whose parent value is a list but whose next key is a non-integer string raises a raw TypeError: list indices must be integers or slices, not str, even though the same assignment over a scalar/None value overrides it with a new dict, and reading the same keypath raises the library's own KeyError.

A list cannot hold a string key, so it is now overridden the same way a scalar is, keeping keypath assignment consistent. Valid integer indexes still address the list unchanged. The fix is at the single navigation choke point, so subset/move/unflatten are covered too.

from benedict import benedict
b = benedict({'a': [1, 2]})
b['a.x'] = 1        # was: TypeError; now: {'a': {'x': 1}}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.38%. Comparing base (a922d1a) to head (b5dd5fa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #591   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          64       64           
  Lines        2418     2419    +1     
=======================================
+ Hits         2379     2380    +1     
  Misses         39       39           

Flag	Coverage Δ
unittests	98.38% <100.00%> (+<0.01%)	⬆️

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