backport: backport low pt electrons to 0.7

[ikrommyd]

Backport of #1121 to coffea 0.7.x.
The same validation:

In [1]: from coffea.nanoevents import NanoEventsFactory

In [2]: events = NanoEventsFactory.from_root("root://eoscms.cern.ch:1094//eos/cms//store/mc/Run3Winter24NanoAOD/DYJetsToLL_M-50_TuneCP5_13p6TeV-madgraphMLM-pythia8/NANOAODSIM/NoPU_Pilot_133X_mcRun3_2024_realistic_v6-v2/50000/976308db-41c9-4b50-b297-d2577ef22e4b.root").events()

In [3]: events.LowPtElectron
Out[3]: <LowPtElectronArray [[], [LowPtElectron, ... [], []] type='1941 * var * lowPtEle...'>

In [4]: events.LowPtElectron.matched_gen
Out[4]: <GenParticleArray [[], [GenParticle, ... [], []] type='1941 * var * ?genParticle'>

In [5]: events.LowPtElectron.matched_electron
Out[5]: <ElectronArray [[], [Electron, Electron, ... [], []] type='1941 * var * ?electron'>

In [6]: events.LowPtElectron.matched_photon
Out[6]: <PhotonArray [[], [None, None], ... None], [], []] type='1941 * var * ?photon'>

In [7]: events.LowPtElectron.metric_table(events.Electron)
Out[7]: <Array [[], [[2.77, 0], [0, ... [4.2]], [], []] type='1941 * var * var * float32'>

[ikrommyd]

@lgray I think this is mergeable