ENH: Add annotations for np.rec

[BvB93]

This PR adds annotations for the np.rec functions as well as the classes that it exports to the main numpy namespace.

In general, the dtype produced by np.rec-based classes and functions can be specified in one of two ways:

• Directly via the dtype argument.
• With up to five helper arguments that operate via np.format_parser: formats, names, titles, aligned and byteorder
  

  numpy/numpy/core/records.py

  Lines 422 to 425 in e0d4bcf

  	if dtype is not None:
  	descr = sb.dtype(dtype)
  	else:
  	descr = format_parser(formats, names, titles, aligned, byteorder).dtype

These two approaches are currently typed as being mutually exclusive, i.e. if dtype is specified than one may not specify formats. While this mutual exclusivity is not (strictly) enforced during runtime, combining both dtype specifiers can lead to unexpected or even downright buggy behavior; enough reason to activelly discourage it, in my opinion.

[charris]

@BvB93 OT: Note that LGTM has an alert for mypy_plugin.py, https://lgtm.com/projects/g/numpy/numpy/alerts/?mode=list&severity=error.

[BvB93]

Seems to be a false positive, the relevant branch of code can only be reached if the to-be raised exception is not None:

if t.TYPE_CHECKING or MYPY_EX is None:
    ...
else:
    def plugin(version: str) -> t.Type[_NumpyPlugin]:
        """An entry-point for mypy."""
        raise MYPY_EX  # LGTM: Illegal class 'NoneType' raised; will result in a TypeError being raised instead.

[charris]

Thanks Bas.