This PR implements comprehensive type hints for the Twilio Python API library to enable proper type checking support with tools like mypy, pyright, and other static type checkers.

Problem

The Twilio Python library previously lacked comprehensive type hints, making it difficult for developers to get proper type checking support and IDE autocompletion. This led to:

• No static type checking capabilities
• Poor IDE support with missing autocompletion
• Increased likelihood of runtime errors that could be caught at development time
• Lack of self-documenting code through type annotations

Solution

This PR adds type hints to core user-facing modules while maintaining backward compatibility:

Infrastructure Changes

• Added twilio/py.typed marker file for PEP 561 compliance
• Created mypy.ini configuration for type checking
• Updated MANIFEST.in to include py.typed in package distributions

Type Hints Added

• Authentication modules: Complete type annotations for AuthType, AuthStrategy, NoAuthStrategy, TokenAuthStrategy
• Credential providers: Type hints for CredentialProvider, ClientCredentialProvider, OrgsCredentialProvider
• Token management: Fixed interface mismatches and added proper type hints for token managers
• Request validation: Comprehensive type hints for RequestValidator with proper Union type handling
• Serialization utilities: Type hints for core serialization functions (iso8601_date, boolean_to_string, etc.)

Example Usage

from twilio.rest import Client
from twilio.auth_strategy.auth_type import AuthType
from twilio.request_validator import RequestValidator

# Type hints provide better IDE support and catch errors
client: Client = Client("account_sid", "auth_token")

# Proper type checking for request validation
validator: RequestValidator = RequestValidator("auth_token")
is_valid: bool = validator.validate(
    uri="https://example.com/webhook",
    params={"key": "value"},
    signature="expected_signature"
)

Documentation

Updated README.md with a comprehensive "Type Checking Support" section that includes:

• Instructions for using mypy and other type checkers
• Example code demonstrating proper usage with type hints
• Reference to PEP 561 compliance

Validation

• All modified modules pass mypy type checking without errors
• Existing unit tests continue to pass, ensuring backward compatibility
• Manual testing confirms imports and basic functionality work correctly
• Created validation scripts to test type hints work for end users

Scope

This implementation focuses on core, user-facing APIs that developers interact with most frequently. The auto-generated API files already had good type coverage. Future work can extend type hints to additional modules as needed.

Fixes #18.

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