[clang] [libclang/python] Ignore our own DeprecationWarnings in CMake targets (PR #178631)

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

These are warnings, not exceptions, so they don't raise.
But the context manager keeps a list of all caught warnings, so I've used that to assert that we got deprecation warnings where expected.

https://github.com/llvm/llvm-project/pull/178631