Add asynchronous context manager support

[Copilot]

This PR adds support for using time_machine.travel as an async context manager, enabling the following usage pattern:

import time_machine

async def test_function():
    async with time_machine.travel("1970-01-01 00:00 +0000"):
        # Time is now mocked to epoch
        assert time.time() < 1.0

Implementation

Added two async context manager methods to the travel class:

• async def __aenter__(self) -> Coordinates: Returns self.start()
• async def __aexit__(...): Calls self.stop()

These methods mirror the existing sync context manager methods (__enter__ and __exit__) but are async-compatible. The implementation delegates to the same underlying start() and stop() methods, ensuring consistent behavior between sync and async usage.

Features

• Full async context manager protocol support
• Returns Coordinates object for time manipulation within the async context
• Works with all existing travel() parameters (tick, destination types, etc.)
• Maintains compatibility with existing sync context manager usage
• Supports coordinate manipulation (shift(), move_to()) within async contexts

Tests

Added comprehensive tests covering:

• Basic async context manager functionality
• Proper cleanup and time restoration after context exit
• Coordinate manipulation within async contexts
• Compatibility with existing async decorator and sync context manager usage

Documentation

Updated the usage documentation to include async context manager examples and mention async support in the main description.

All existing tests continue to pass, confirming no regressions were introduced.

Fixes #478.

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[adamchainz]

@copilot remember to update docs/usage.rst and docs/changelog.rst

[Copilot]

@copilot remember to update docs/usage.rst and docs/changelog.rst

Updated both documentation files:

• docs/usage.rst: Already had async context manager examples and mentions (from previous commit)
• docs/changelog.rst: Added entry for async context manager support in version 2.18.0 with reference to issue Add async context manager support #478

Commit: 22e143e