Feature/replace hyper with httpx

[paulskeie]

Replace hyper with httpx for HTTP/2 client

This PR replaces the hyper HTTP/2 client library with httpx. The main changes are:

Changes

• Replace hyper dependency with httpx (>=0.24.0)
• Update client connection handling to use httpx Client
• Update credentials module to use httpx for connections
• Update tests to mock httpx instead of hyper
• Add Python 3.10-3.12 support
• Set explicit Python version range (>=3.7,<4.0)

Testing

All tests pass with the new httpx implementation:

• Unit tests updated to mock httpx client
• Integration tests working with APNs sandbox
• Token and certificate auth both functional

Migration Notes

• No API changes required for users
• Same functionality maintained
• Better HTTP/2 support through httpx
• Improved connection handling
• More robust error handling

Dependencies

• Added httpx>=0.24.0
• Removed hyper
• Updated Python version support (3.7-3.12)