Integrate into Cython test suite later

GH-2919: Prevent tuple constants that compare equal but have different item types from being merged into one.

GH-2934: improve error reporting for non-constant exception value expressions

GH-2934: improve error reporting for non-constant exception value expressions: instead of two errors and a crash, just report it once and explicitly.

Clean weakrefs before calling user's dealloc

Add PyGILState_Check to Includes/cpython/pystate.pxd

Closes #2926.