Create companion methods of "PyType_FromSpec*" to allow setting metaclass.

[WildCard65]

BPO	41171
Nosy	@WildCard65
PRs	bpo-41171: Added method "PyMetaType_FromSpec" to allow for specifying the metaclass for the type. #21238

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-30.15:34:35.127>
labels = ['expert-C-API', 'type-feature', '3.8', '3.9', '3.10']
title = 'Create companion methods of "PyType_FromSpec*" to allow setting metaclass.'
updated_at = <Date 2020-06-30.16:29:58.229>
user = 'https://github.com/WildCard65'

bugs.python.org fields:

activity = <Date 2020-06-30.16:29:58.229>
actor = 'WildCard65'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2020-06-30.15:34:35.127>
creator = 'WildCard65'
dependencies = []
files = []
hgrepos = []
issue_num = 41171
keywords = ['patch']
message_count = 2.0
messages = ['372696', '372697']
nosy_count = 1.0
nosy_names = ['WildCard65']
pr_nums = ['21238']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41171'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[WildCard65]

The current goal from what I can tell for Python is to have all C based modules move away from static types and instead use "PyType_FromSpec" and the variant that specifies base classes.

The only problem is, PyType_FromSpec and it's variant makes the assumption the caller wants "PyType_Type" as the type's metaclass.

Why not add companion methods to them prefixed with "PyMetaType" and have the "PyType" ones internally invoke these new methods with "PyType_Type" as the metaclass (to keep existing behavior and backwards compatibility)

[WildCard65]

Another thing I thought of, if this is accepted, we can turn the "PyType" methods into header static inline methods.

[kumaraditya303]

Duplicate of #60074