tools: ynl: avoid dict errors on older Python versions

Python 3.9.0 or newer supports combining dicts() with |,
but older versions of Python are still used in the wild
(e.g. on CentOS 8, which goes EoL May 31, 2024).
With Python 3.6.8 we get:

  TypeError: unsupported operand type(s) for |: 'dict' and 'dict'

Use older syntax. Tested with non-legacy families only.

Fixes: f036d936

 ("tools: ynl: Add fixed-header support to ynl")
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Tested-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230524170712.2036128-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>