[Python][UHI] Fix entry count after slicing operations

[siliataider]

This Pull request:

Changes or fixes:

When using UHI for slicing, GetEntries() value of a histogram did not correctly reflect the state after the operation.

Note that after this fix, calling GetEntries() on a sliced histogram will reflect the effective total count (the same value returned by GetEffectiveEntries()).

The following:

import ROOT

h = ROOT.TH1D("h", "h", 10, 0, 10)
h.SetBinContent(1, 5)

h_slice = h[...]

h.Print()
h_slice.Print()

Will output:

TH1.Print Name  = h, Entries= 1, Total sum= 5
TH1.Print Name  = h, Entries= 5, Total sum= 5

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #19113

[hageboeck]

This looks reasonable to me, thanks! 👍

[github-actions]

Test Results

    20 files      20 suites   4d 1h 16m 8s ⏱️
 3 013 tests  3 006 ✅   0 💤 7 ❌
58 672 runs  58 509 ✅ 156 💤 7 ❌

For more details on these failures, see this check.

Results for commit 7ff6013.

♻️ This comment has been updated with latest results.

[vepadulano]

LGTM thanks!