micropython: update to 1.28.0

micropython: update to 1.28.0

Changes since 1.27.0:
- New machine.CAN class with bindings for the stm32 port; support across
  all ports to follow
- machine.PWM support added to stm32 and alif ports, completing coverage
  of all Tier 1/2 MCU-based ports
- Template strings (t-strings, PEP 750) added at the "full feature" level
- weakref module added with weakref.ref and weakref.finalize classes
- f-strings now support nested f-strings within expressions
- Optimisations to native emitter; new RISC-V Zcmp arch flag for RV32
- extmod.mk: add extmod/machine_can.c (shifts the mbedtls hunk by 1 line;
  update 040-extmod-use-external-mbedtls.patch accordingly)

micropython-lib is updated in lockstep in a separate commit.

Ref: https://github.com/micropython/micropython/releases/tag/v1.28.0
Signed-off-by: Alexandru Ardelean <redacted>