In new hardware API, these classes implement master modes of interfaces,
and "mode" parameter is not accepted. Trying to implement new HW API
in terms of older pyb module leaves variuos corner cases:
In new HW API, I2C(1) means "I2C #1 in master mode" (? depends on
interpretation), while in old API, it means "I2C #1, with no settings
changes".
For I2C class, it's easy to make mode optional, because that's last
positional param, but for SPI, there's "baudrate" after it (which
is inconsistent with I2C, which requires "baudrate" to be kwonly-arg).
