py: Only store the exception instance on Py stack in bytecode try block.
When an exception is raised and is to be handled by the VM, it is stored on the Python value stack so the bytecode can access it. CPython stores 3 objects on the stack for each exception: exc type, exc instance and traceback. uPy followed this approach, but it turns out not to be necessary. Instead, it is enough to store just the exception instance on the Python value stack. The only place where the 3 values are needed explicitly is for the __exit__ handler of a with-statement context, but for these cases the 3 values can be extracted from the single exception instance. This patch removes the need to store 3 values on the stack, and instead just stores the exception instance. Code size is reduced by about 50-100 bytes, the compiler and VM are slightly simpler, generate bytecode is smaller (by 2 bytes for each try block), and the Python value stack is reduced in size for functions that handle exceptions.