[Culaii]

Refs #1113, #1131. Complements #1114 (__all__), does not depend on it.

Problem

Since 3.3.3 ships py.typed, a project running mypy --strict (disallow_untyped_calls)
gets one error per declaration it writes, because the declarations' constructors are
unannotated:

error: Call to untyped function "SubFactory" in typed context  [no-untyped-call]
error: Untyped decorator makes function "with_tags" untyped  [untyped-decorator]

(the first one was reported in #1113). On a real Django project with ~30 factory modules
this is 138 errors, all of them about factory_boy rather than about the project; the usual
workaround is disallow_untyped_calls = false for the whole package.

Change

Annotate what users call:

• the constructors of every declaration exported by factory (Sequence, SubFactory,
  LazyAttribute, Trait, Maybe, RelatedFactory, PostGeneration, ...),
  factory.Faker and factory.django.Password;
• factory.helpers: build/create/... and the decorators (post_generation,
  lazy_attribute, sequence, ...);
• the public classmethods of BaseFactory (stub, generate, simple_generate,
  reset_sequence, ...);
• factory.django.mute_signals, so that a decorated function keeps its signature.

What users override (evaluate, call, _create, _build, _adjust_kwargs, ...) is left
untouched on purpose: annotating it would make mypy check user overrides for compatibility.

Why this should not break anyone (cf. #1113)

• The type of a declaration expression does not change: mypy already inferred
  factory.Sequence(...) as Sequence; only the arguments are now checked.
• mypy does not check __init__ overrides for compatibility, so subclasses of declarations
  are not affected.
• tests/test_typing_unannotated.py type-checks the documented, unannotated style
  (@factory.sequence def username(n): ... on a factory). This is why the sequence
  decorator accepts Callable[[Any], Any]: on an unannotated method mypy types n as the
  factory class, and Callable[[int], Any] would have reported a new error there.
• No runtime change; the suite passes unchanged.

Tests

• make test now also runs
  mypy --strict --implicit-reexport --follow-imports=silent tests/test_typing.py tests/test_typing_unannotated.py,
  i.e. sees factory the way a strict downstream project does. --implicit-reexport can be
  dropped once Explicitly export values on factory namespace #1114 lands.
• tests/test_typing.py builds a factory using every annotated declaration (also checked at
  runtime), and contains wrong calls marked # type: ignore[...]: under --strict,
  warn_unused_ignores fails if an annotation stops rejecting them.
• On master's factory/, the strict run reports 27 errors on these tests; with this change, 0.

Out of scope (happy to follow up)

• UserFactory() still returns UserFactory for type checkers (Support type hinting? #468, typing: fix BaseFactory callable return type for parameterized generics. (Works in VSCode only) #1137).
• Generic declarations (Sequence[str], as in types-factory-boy), factory.fuzzy,
  Faker.add_provider / override_default_locale, generic ORM factories (Add missing generic on Factory subclasses #1060, feat: Add generic typing to factories #1100).
• Side note: mypy factory reports factory/random.py:7: "Random" has no attribute "state_set"
  on master; CI doesn't see it because tests/test_typing.py doesn't import factory.random.

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