Fix support for comparing key columns of different names (via the Python API)

[erezsh]

Describe the bug

When comparing two TableSegment with different key columns, the differ fails with an error:

    426         for c in table1._relevant_columns:
    427             if c not in table1._schema:
--> 428                 raise ValueError(f"Column '{c}' not found in schema for table {table1}")
    429             if c not in table2._schema:
    430                 raise ValueError(f"Column '{c}' not found in schema for table {table2}")

[erezsh]

Fixed.