gh-140795: Remove 'exc' field in SSLObject (gh-143491)

gh-140795: Remove 'exc' field in SSLObject (gh-143491)

The 'exc' field was used by our debug SSL callbacks. Keep the exception
in the normal per-thread state to avoid shared mutable state between
threads.

This also avoids a reference count leak if the Python callback raised an
exception because it can be called multiple times per SSL operation.