Use a copy of `self.contents` in this list comprehension.

Use a copy of `self.contents` in this list comprehension.

Improved a fix first implemented for :ticket:`3208` released in version
0.9.8, where the registry of classes used internally by declarative could
be subject to a race condition in the case where individual mapped classes
are being garbage collected at the same time while new mapped classes are
being constructed, as can happen in some test suite configurations or
dynamic class creation environments.   In addition to the weakref check
already added, the list of items being iterated is also copied first to
avoid "list changed while iterating" errors.  Pull request courtesy Yilei
Yang.

Fixes: #10782
Closes: #10783
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10783
Pull-request-sha: 354e97b640430120d0c193a4efe487f293d4768b

Change-Id: I04ccc92472bf1004dad0fb785e16b180f58f101d