Preserve user defined slots on attrs classes...

[emadleo13]

Summary

When an attrs class defines __slots__ explicitly, the generated slotted class did not preserve those user-defined slots.

This change preserves explicitly declared __slots__ while avoiding duplicate slots inherited from base classes.

Changes

• Preserve user-defined __slots__ when generating the slotted class.
• Support both tuple-based and string-based __slots__.
• Avoid re-creating slots already inherited from base classes.
• Remove the original slot descriptors from the class dictionary before creating the new class.

Tests

Added regression tests covering:

• User-defined tuple-based slots.
• User-defined string-based slots.
• User-defined slots combined with inherited slots.

Full test suite:

• 1410 passed
• 4 skipped
• 2 x failed