Fix PyUnknownFields memory leak

[agrenott]

Properly release internal data structure on deallocation.
Fix #7301

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[agrenott]

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[agrenott]

I disabled the new memory leak unit test for python versions <3.4 (as tracemalloc wasn't available back then).

[agrenott]

Hi,
Is anything on my side missing to get this PR reviewed/integrated?
This leak's quite a show-stopper on our side.
Thx

[agrenott]

Hi,
Any idea when this PR could get merged (or what's missing)?
Thx.

[dlj-NaN]

Thanks, and sorry for the delay.

[agrenott]

Hi,
Thanks for the merge.
Is there a way to get it merged into the on-going 3.14.x branch? If so what's the process?

[acozzette]

@agrenott Thanks for flagging this, I created #8032 to cherry-pick this fix into the 3.14.x branch.