## Change Summary

This means `pydantic/__init__.py` is now lazy about all imports using `__getattr__`.

Import performance changes:
### Before

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 29.7 msec per loop
```

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"
1 loop, best of 1: 30.4 msec per loop
```

### After

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 4.8 msec per loop
```

```
(env3112) code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"           
1 loop, best of 1: 18.7 msec per loop
```

Tuna output:

<img width="1421" alt="image" src="https://github.com/pydantic/pydantic/assets/4039449/0a57434e-fdda-4709-8793-d96c0a0ee762">

## Related issue number

As promised in https://github.com/pydantic/pydantic/pull/7423#issuecomment-1733361629.

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [ ] Unit tests for the changes exist
* [x] Tests pass on CI
* [ ] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @sydney-runkle
Use `__getattr__` for all package imports, improve import time. ## Change Summary

This means `pydantic/__init__.py` is now lazy about all imports using `__getattr__`.

Import performance changes:
### Before

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 29.7 msec per loop
```

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"
1 loop, best of 1: 30.4 msec per loop
```

### After

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 4.8 msec per loop
```

```
(env3112) code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"           
1 loop, best of 1: 18.7 msec per loop
```

Tuna output:

<img width="1421" alt="image" src="https://github.com/pydantic/pydantic/assets/4039449/0a57434e-fdda-4709-8793-d96c0a0ee762">

## Related issue number

As promised in https://github.com/pydantic/pydantic/pull/7423#issuecomment-1733361629.

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [ ] Unit tests for the changes exist
* [x] Tests pass on CI
* [ ] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @sydney-runkle
from pydantic import BaseModel, Field


class ConfigClassUsed(BaseModel):
    i: int = Field(2, alias='j')

    class Config:
        populate_by_name = True


ConfigClassUsed(i=None)
# MYPY: error: Argument "i" to "ConfigClassUsed" has incompatible type "None"; expected "int"  [arg-type]


class MetaclassArgumentsNoDefault(BaseModel, populate_by_name=True):
    i: int = Field(alias='j')


MetaclassArgumentsNoDefault(i=None)
# MYPY: error: Argument "i" to "MetaclassArgumentsNoDefault" has incompatible type "None"; expected "int"  [arg-type]


class MetaclassArgumentsWithDefault(BaseModel, populate_by_name=True):
    i: int = Field(2, alias='j')


MetaclassArgumentsWithDefault(i=None)
# MYPY: error: Argument "i" to "MetaclassArgumentsWithDefault" has incompatible type "None"; expected "int"  [arg-type]


class NoArguments(BaseModel):
    i: int = Field(2, alias='j')


NoArguments(i=1)
# MYPY: error: Unexpected keyword argument "i" for "NoArguments"  [call-arg]
NoArguments(j=None)
# MYPY: error: Argument "j" to "NoArguments" has incompatible type "None"; expected "int"  [arg-type]

from pydantic import BaseModel, Field


class ConfigClassUsed(BaseModel):
    i: int = Field(2, alias='j')

    class Config:
        populate_by_name = True


ConfigClassUsed(i=None)
# MYPY: error: Argument "i" to "ConfigClassUsed" has incompatible type "None"; expected "int"  [arg-type]


class MetaclassArgumentsNoDefault(BaseModel, populate_by_name=True):
    i: int = Field(alias='j')


MetaclassArgumentsNoDefault(i=None)
# MYPY: error: Argument "i" to "MetaclassArgumentsNoDefault" has incompatible type "None"; expected "int"  [arg-type]


class MetaclassArgumentsWithDefault(BaseModel, populate_by_name=True):
    i: int = Field(2, alias='j')


MetaclassArgumentsWithDefault(i=None)
# MYPY: error: Argument "i" to "MetaclassArgumentsWithDefault" has incompatible type "None"; expected "int"  [arg-type]


class NoArguments(BaseModel):
    i: int = Field(2, alias='j')


NoArguments(i=1)
# MYPY: error: Unexpected keyword argument "i" for "NoArguments"  [call-arg]
NoArguments(j=None)
# MYPY: error: Argument "j" to "NoArguments" has incompatible type "None"; expected "int"  [arg-type]
