Release v3.1.0a2

Added

• Support for reading button state on Technic Hub, City Hub, and Move Hub.
• Support for reading hub name on all Powered Up hubs.
• Enable MicroPython slicing feature on Technic Hub, City Hub, Prime Hub.
• Added pystone benchmark tests for all hubs.
• Added hub.system.shutdown() for graceful shutdown (support#58).

Changed

• Move Remote class from experimental to pupdevices.
• Move Remote.pressed() to Remote.buttons.pressed().
• Remove pybricks.experimental module on Move Hub.
• Increased Move Hub heap from 6K to 7K due to recently freed RAM (pybricks-micropython#57).
• Changed hub.system.reset(action) to accept 2 only (support#379). This
  is technically a breaking change, although this method was never officially
  released or documented. With this change, entering firmware update mode
  remains possible for backwards-compatibility, but unsafe power off methods
  are removed.

Fixed

• Fix Bluetooth not disconnecting when shutting down and button is still pressed.
• Fix I/O ports not powering off when shutting down and button is still pressed.
• Fix version number only showing git hash and not v3.x.