feat: setup basedpyright

[Paillat-dev]

In an effort to have better typing, this sets up basedpyright with the "baseline" feature, which will ignore all typing errors from before this pull request and allow for gradual typing of code. Idk if this is the best, but I figured I might as well try. This needs #4 to be merged first.

[Copilot]

Pull Request Overview

This pull request adds a basedpyright CI job with a “baseline” configuration and modernizes type annotations across the codebase by replacing Union[...] with the new | syntax, plus a couple of strict-parameter adjustments in zip calls.

• Introduce basedpyright run in GitHub Actions for gradual typing
• Replace typing.Union annotations with PEP 604-style unions (A | B)
• Add explicit strict=False to zip in two methods (though default is already False)
• Update changelog to note removal of Python 3.9 support

Reviewed Changes

Copilot reviewed 38 out of 38 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
discord/state.py	Updated Channel union syntax; added strict=False to zip in _delay_ready
discord/role.py	Added strict=False to zip in _move payload comprehension
discord/object.py	Updated SupportsIntCast alias to PEP 604 union syntax
discord/message.py	Modernized EmojiInputType to use `
discord/member.py	Replaced VocalGuildChannel Union with `
discord/iterators.py	Replaced internal _Func return union with `
discord/invite.py	Swapped out Union[...] for TypeAlias-style unions
discord/interactions.py	Modernized InteractionChannel union syntax
discord/guild.py	Converted channel-type aliases to PEP 604 unions
discord/ext/commands/flags.py	Adjusted optional flag handling; removed import of Union
discord/ext/commands/context.py	Updated BotT bound to `Bot
discord/ext/commands/_types.py	Converted many Union-based definitions to `
discord/ext/bridge/context.py	Simplified Context union to `
discord/errors.py	Replaced _ResponseType union with PEP 604 syntax
discord/commands/options.py	Refactored InputType to new union grouping syntax
discord/commands/core.py	Marked deprecated Union[...] usage with TODO and noqa
discord/activity.py	Updated ActivityTypes to use PEP 604 unions
discord/abc.py	Cleaned up MessageableChannel and SnowflakeTime aliases
CHANGELOG-V3.md	Noted removal of Python 3.9 support
.github/workflows/lib-checks.yml	Added basedpyright job to CI workflow

Comments suppressed due to low confidence (1)

discord/ext/commands/flags.py:421

• You removed the Union import at the top but still reference Union here, which will cause a NameError. Re-import Union or replace this with the modern | syntax.

annotation = Union[annotation.__args__[:-1]]  # noqa: UP007