kconfiglib: update to the latest version

Corresponds to ba71a0e (Fix _parse_block() 'parent' documentation re.
ifs.) from upstream, just adding the SPDX tag.

Has performance improvements, code cleanup, Python 3 support, and various
small fixes, including the following:

  - Unset user values when loading a zero-byte .config. (5e54e2c)
  - Ignore indented .config assignments. (f8a7510)
  - Do not require $srctree to be set for non-kernel projects. (d56e9c1)
  - Report correct locations in the presence of continuation lines.
    (0cebc87)
Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>