Fix Python 3.9 compatibility by replacing union syntax with typing.Optional

[Copilot]

The recent updates in v2.0.9 introduced Python 3.10+ union type syntax (str | None) in function parameter annotations, which causes a TypeError when importing PyFCM on Python 3.9:

from pyfcm import FCMNotification
# TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

This occurs because the union operator | for type hints was only introduced in Python 3.10. The error happens in baseapi.py when Python tries to evaluate the type annotations in the BaseAPI.__init__() method.

Changes:

• Added from typing import Optional import to pyfcm/baseapi.py
• Replaced all instances of Type | None with Optional[Type] in the BaseAPI constructor:
  • service_account_file: str | None → Optional[str]
  • project_id: str | None → Optional[str]
  • credentials: Credentials | None → Optional[Credentials]
  • proxy_dict: dict | None → Optional[dict]
  • env: str | None → Optional[str]

The fix uses typing.Optional, which is compatible with Python 3.5+ and maintains the exact same API and functionality. All existing tests continue to pass.

Fixes #375.

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[Copilot]

Pull Request Overview

This PR fixes Python 3.9 compatibility by replacing the union type syntax (|) introduced in Python 3.10 with the backward-compatible typing.Optional syntax in function parameter annotations.

• Adds from typing import Optional import to maintain Python 3.5+ compatibility
• Replaces all Type | None union syntax with Optional[Type] in BaseAPI constructor parameters
• Ensures PyFCM can be imported without errors on Python 3.9 and earlier versions