SyntaxError: from __future__ imports must occur at the beginning of the file when custom header is used

[Dezzley]

Describe the bug
When a model is generated with a custom header, importing the resulting model module causes the following error:

SyntaxError: from __future__ imports must occur at the beginning of the file

This error occurs because the from __future__ statement must appear near the top of the module (see documentation)

To Reproduce
Generate model via the provided command below using any schema:

Used commandline:

$  datamodel-codegen --input "./openapi.yml" --input-file-type openapi --output ./model.py --output-model-type pydantic_v2.BaseModel --snake-case-field --target-python-version 3.10 --use-schema-description --use-field-description  --base-class CustomBaseModel --custom-file-header-path ./custom_pydantic_file_header.py

Expected behavior
Generated module could be imported without any issues.

Version:

• OS: MacOS Sonoma 14.6.1
• Python version: 3.10.14
• datamodel-code-generator version: 0.25.9

Additional context
Consider hoisting the future statement if any custom header option is passed.