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CI has passed. Ready for folks to take a look.

Outline of changes, to hopefully make the review easier:

• There are two kinds of TypedDict declarations mypy recognizes, a class-based form and an assignment-based form:

class Movie(TypedDict, total=False):  # class-based
    title: Required[str]
    year: int

Movie = TypedDict('Movie', {  # assignment-based
    'title': Required[str],
    'year': int,
}, total=False)

• Those declarations are parsed by, respectively:
  • TypedDictAnalyzer.analyze_typeddict_classdef @ mypy/semanal_typeddict.py
  • SemanticAnalyzer.analyze_typeddict_assign @ mypy/semanal.py
• This commit alters both of these functions to recognize Required[...] as valid wrapper around each key's type, and to treat such-marked keys as required.
• Both of these functions obtain a Type for each key in the TypedDict by calling SemanticAnalyser.anal_type() on the key type.
• anal_type() is changed by this commit to recognize Required[...] as a RequiredType in:
  • try_analyze_special_unbound_type @ mypy/typeanal.py
• We only want to allow the Required[...] syntax in the very narrow situation of a TypedDict key definition. So the top call of SemanticAnalyser.anal_type() takes an allow_required=True parameter that can be used to enable recogition of Required[...] by the two TypedDict-parsing methods, but to disable recognition by default in other contexts.

I did a quick review pass and overall looks good. It's okay to move forward with supporting NotRequired as an extension.

Thanks for the review @JukkaL !

I'll convert this PR to draft for now and add additional commits for the NotRequired case. Separately, I'll make a PR for the typing_extensions module.

PR is extended with support for NotRequired[] and is ready for another round of review.

Would you be able to fix the merge conflict -- I can review this now. Apologies for the very slow response!

I’ve been disconnected from open source for several months to take care of some life issues. I’m hoping I’ll have enough energy to reengage here in a month or so.

In the meantime I welcome any rebase/deconflict support from any observers who’d like this branch to be merged faster.

No problem!

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/iterators.py:70: error: Name "Required" is not defined  [name-defined]
- steam/iterators.py:71: error: Name "Required" is not defined  [name-defined]
- steam/trade.py:70: error: Variable "typing_extensions.Required" is not valid as a type  [valid-type]
- steam/trade.py:70: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
- steam/trade.py:71: error: Variable "typing_extensions.Required" is not valid as a type  [valid-type]
- steam/trade.py:71: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
- steam/trade.py:275: error: Argument 1 to "update" of "TypedDict" has incompatible type "AssetDict"; expected "TypedDict({'instanceid'?: Required?[builtins.str], 'classid'?: Required?[builtins.str], 'market_name'?: str, 'currency'?: int, 'name'?: str, 'market_hash_name'?: str, 'name_color'?: str, 'background_color'?: str, 'type'?: str, 'descriptions'?: Dict[str, str], 'market_actions'?: List[Dict[str, str]], 'tags'?: List[Dict[str, str]], 'actions'?: List[Dict[str, str]], 'icon_url'?: str, 'icon_url_large'?: str, 'tradable'?: bool, 'marketable'?: bool, 'commodity'?: int, 'fraudwarnings'?: List[str]})"  [typeddict-item]
+ steam/trade.py:275: error: Argument 1 to "update" of "TypedDict" has incompatible type "AssetDict"; expected "TypedDict({'instanceid'?: str, 'classid'?: str, 'market_name'?: str, 'currency'?: int, 'name'?: str, 'market_hash_name'?: str, 'name_color'?: str, 'background_color'?: str, 'type'?: str, 'descriptions'?: Dict[str, str], 'market_actions'?: List[Dict[str, str]], 'tags'?: List[Dict[str, str]], 'actions'?: List[Dict[str, str]], 'icon_url'?: str, 'icon_url_large'?: str, 'tradable'?: bool, 'marketable'?: bool, 'commodity'?: int, 'fraudwarnings'?: List[str]})"  [typeddict-item]
- steam/http.py:70: error: Name "Required" is not defined  [name-defined]
- steam/http.py:71: error: Name "Required" is not defined  [name-defined]
- steam/state.py:70: error: Name "Required" is not defined  [name-defined]
- steam/state.py:71: error: Name "Required" is not defined  [name-defined]

@JukkaL Thanks for your review! And this PR also looks good to me.

Thanks @97littleleaf11 for getting this branch finished while I've had to step back for a while! And thank you @JukkaL for getting it reviewed. Team work :)