Fix clang ubsan (undefined behavior sanitizer) warnings in dictobject.c by
adjusting how the internal struct _dictkeysobject shared keys structure is
declared.

This remains ABI compatible.  We get rid of the union at the end of the
struct being used for conveinence to avoid typecasting in favor of char[]
variable length array at the end of a struct. This is known to clang to be
used for variable sized objects and will not cause an undefined behavior
problem.  Similarly, char arrays do not have strict aliasing undefined
behavior when cast.

PEP-007 does not currently list variable length arrays (VLAs) as allowed
in our subset of C99.  If this turns out to be a problem, the fix to this is
to change the char `dk_indices[]` into `dk_indices[1]` and restore the
three size computation subtractions this change removes:
  `- Py_MEMBER_SIZE(PyDictKeysObject, dk_indices)`

If this works as is I'll make a separate PR to update PEP-007.