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This allows to efficiently send to an I2C slave data that is made up of
more than one buffer.  Instead of needing to allocate temporary memory to
combine buffers together this new method allows to pass in a tuple or list
of buffers.  The name is based on the POSIX function writev() which has
similar intentions and signature.

The reasons for taking this approach (compared to having an interface with
separate start/write/stop methods) are:
- It's a backwards compatible extension.
- It's convenient for the user.
- It's efficient because there is only one Python call, then the C code can
  do everything in one go.
- It's efficient on the I2C bus because the implementation can do
  everything in one go without pauses between blocks of bytes.
- It should be possible to implement this extension in all ports, for
  hardware and software I2C.

Further discussion is found in issue #3482, PR #4020 and PR #4763.