fix: make fontsize same for experiment label and secondary label for ATLAS

[diptaparna]

The font sizes for "ATLAS" and "Simulation/Internal/Preliminary" needs to be the same as per the latest ATLAS plot style.
However, in mplhep, the font size for "ATLAS" was intentionally 1.3x the font size of the secondary label.
This pull request fixes this issue.

[ntadej]

I confirm this from the ATLAS side.

[matthewfeickert]

Why are these needed?

[diptaparna]

They get created when you run pytest.

[diptaparna]

Can we please merge this?

[andrzejnovak]

Ok, thanks for the fix! Since I am not ATLAS, I was waiting for some contacts to corroborate the change.

[andrzejnovak]

The release should appear on pypi shortly @diptaparna

[matthewfeickert]

Relevant details from the Style guide for ATLAS Plots:

ATLAS labels

Use the ATLAS labels if and only if your plot deserves them. Rules that govern the use of the ATLAS labels can be found in the Physics Policy Document PDF. If your plot has not been approved by ATLAS to be shown outside the Collaboration, it should not have an "ATLAS" or "ATLAS Preliminary" label.

• The ATLAS name must be draw in Helvetica, all upper case, bold face, slanted. The ATLAS Style package provides a macro to do it.
• Any additional text after the ATLAS name (e.g. "Preliminary") must be drawn in Helvetica normal typeface (thus, neither bold nor slanted), with first letter capitalized, in the same font size of the ATLAS name. The ATLAS Style has an example on how to properly do it.

This information would have been useful to include in the commit message itself.

---

Thanks for the PR @diptaparna and thanks for your first contribution to Scikit-HEP! 🎉