main/py3-cparser: upgrade to 2.21

https://github.com/eliben/pycparser/blob/release_v2.21/CHANGES

release_v2.20...release_v2.21

+ Version 2.21 (2021.11.06)

  - Much improved support for C11 (multiple PRs)
  - Support for parehthesized compount statements (#423)
  - Support for modern Python versions (3.9 and 3.10)
  - Fix support for structs with nested enums (#387)
  - Multiple small bug fixes