bug: flattening a list of strings result in a ValueError (recursive elements)

[qexat]

Describe the bug
Flattening a list of strings result in a ValueError: cannot flatten list because it contains recursive elements.

This is because strings are Sequences, so the algorithm will try to flatten them further, but they never end up being a non-Sequence (single-char strings are still strings) so it loops forever.

To Reproduce
Steps to reproduce the behavior:

from magic_list import L

l = L["hello", "world"]
print(l.flatten())  # ValueError: cannot flatten list because it contains recursive elements

Expected behavior
The expected behavior is debatable. It should either give the list untouched, or flatten it to a list of characters.

Configuration:

• Python version: all supported
• Magic List version: all since flatten() was introduced

Additional context
Found after reading https://discuss.python.org/t/collections-abc-nonstringsequence/53274

[qexat]

Warning

The fix will be a breaking change. flatten will only decrement the list dimension by one, instead of always bringing it to 1.

It will also allow to properly type the method (without using nefarious Anys): indeed, it is a known issue in languages that are not dependently-typed that it is impossible to properly type a flatten function as it is behaving before the fix.

More information: https://github.com/users/qexat/projects/2?pane=issue&itemId=70052105