FIX: set colours for Verbatim without using private Sphinx macros

[jfbu]

This avoids using private Sphinx macro \spx@verbatimfcolorbox which got renamed at Sphinx 5.1.0 and whose definition would thus remain without effect.

Fixes: #101

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[jfbu]

In second commit I removed \makeatletter/\makeatother as @ is not used.

But anyhow as this is a .cls file so a priori if used normally (i.e. not directly via \input but via \documentclass) then the @ needed no \makeatletter/\makeatother wrapper to start with anyhow.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.21%. Comparing base (6785357) to head (90d4bd5). Report is 9 commits behind head on master.

Additional details and impacted files

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[AakashGfude]

Thanks @jfbu, this code looks good to me.

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