Fix iterparse XES importer crashing on a nested <list> inside a list-of-values

[mittalpk]

Fixes #566.

What's wrong

pm4py.read_xes()'s actual default variant is iterparse whenever lxml is installed (constants.DEFAULT_XES_PARSER), not chunk_regex as the docstring states. iterparse.py's __parse_attribute() crashes with TypeError: list indices must be integers or slices, not str on any XES file containing a <list> element nested inside another <list>'s <values> block — e.g. a log-level kpi list containing several named sub-lists (OTC-ON, OTC-CN), exactly as in the reported file.

The root cause: when an element's own children container is a list() (because its parent's first child tag was <values>, signaling "this is a list of values, not a dict of named attributes"), any nested <list> element inside it still went through store[key] = {...} — which crashes because store is a list, not a dict. The sibling "leaf value" branch a few lines above already handled this correctly (store.append((key, value)) when store is a list); the "has children" branch just never mirrored that handling for nested lists.

Fix

__parse_attribute() now builds the child-entry dict first, then branches only on whether to .append((key, entry)) (list store) or store[key] = entry (dict store) — reusing the exact pattern already used for leaf values, rather than assuming store is always a dict.

Testing

• Reproduced the exact reported error with a minimal XES file matching the issue's structure (a log-level list containing a nested list with its own nested attributes, and a sibling list containing two nested sub-lists), against the real default (variant='iterparse', i.e. lxml installed). Confirmed chunk_regex and line_by_line do not hit this bug, only iterparse. Fails on unpatched code, passes after the fix (checked via git stash isolation of just the fix file).
• Added test_iterparse_nested_list_inside_a_list_of_values to tests/xes_deep_coverage_test.py, asserting the parsed nested structure matches the source XES exactly.
• Ran xes_deep_coverage_test.py, xes_impexp_test.py (including its "problematic/malformed logs" coverage), facade_utils_coverage_test.py, and main_fac_test.py — 33 tests total, all pass, no regressions.