simplify UART handler, require Micropython version >=1.26

[alustig3]

previously, there was oddness with the pyboard v1.1 (STM32F4) IRQ_RXIDLE
from the UART docs:

On STM32F4xx MCU’s, using the trigger UART.IRQ_RXIDLE the handler will be called once after the first character and then after the end of the message, when the line is idle

This difference in how IRQ_RXIDLE worked on pyboard v1.1 and pyboard d-series led to added complexity in UART_handler class where you had to specify mcu=STM32_F4 or mcu=STM32_F7

The extra trigger after the first character has now been fixed in Micropython v1.26
from release notes:

extmod_hardware/machine_uart_irq_rxidle.py: ignore inital IRQ

This commit has changes that simplify the UART_handler class, with the caveat that it now requires Micropython >= 1.26

[ThomasAkam]

Thanks Andy