adding missing length attribute to ActionPopMPLS

[italovalcy]

Closes #104

Summary

See updated changelog file and/or add any other summarized helpful information for reviewers

Local Tests

> python3
Python 3.13.2 (main, Feb  4 2025, 14:51:09) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pyof.v0x04.common.action import ActionPopMPLS
>>> a = ActionPopMPLS(ethertype=0)
>>> a.pack()
b'\x00\x14\x00\x08\x00\x00\x00\x00'
>>>

End-to-End Tests

Please refer to the results presented here: #110

The end-to-end tests were executed with all PRs stacked

[Alopalao]

LGTM

[viniarck]

Nicely done @italovalcy, thanks for fixing this adjacent issue related to the things you've been working on pyof