bpo-42197: Don't create `f_locals` dictionary unless we actually need it. (GH-32055)

* `PyFrame_FastToLocalsWithError` and `PyFrame_LocalsToFast` are no longer called during profile and tracing.
 (Contributed by Fabio Zadrozny)

* Make accesses to a frame's `f_locals` safe from C code, not relying on calls to `PyFrame_FastToLocals` or `PyFrame_LocalsToFast`.

* Document new `PyFrame_GetLocals` C-API function.