Fix Composition for mixed species & element compositions

[kavanase]

Composition objects initialised as e.g. Composition({"Cd2+": 1, "Cd": 1, "Te2-": 2}) do not behave as expected, giving e.g. Composition.formula = "Cd3 Te2" instead of Cd2 Te2. This is due to Composition.__getitem__() matching all species and elements/non-species of a given input string; so that Composition["Cd"] here would return 2 (as desired), but than Composition.items() (which is use for get_el_amt_dict, formula and more) gives ("Cd2+", 1), ("Cd", 2), ("Te", 2) which is incorrect.
In the end, it's an easy fix to just define the items() method to avoid the Composition.__getitem__() (which is only needed when the user does Composition[...]).

Test added, which failed with previous code and passes now.

Context: Took a while to find what was going on here. Was originally flagged by this ValueError with ComputedStructureEntry: https://github.com/materialsproject/pymatgen/blob/master/src/pymatgen/entries/computed_entries.py#L588

With original pymatgen code:

With fixed code:

Flagged by tests in ShakeNBreak, because defect generation in pymatgen-analysis-defects mixes species with just element symbols; e.g. converting to just element symbol with get_element here: https://github.com/materialsproject/pymatgen-analysis-defects/blob/main/pymatgen/analysis/defects/core.py#L566-L576

[shyuep]

Great thanks.