Make it possible to allow lower case enum names

[libre-man]

Is your feature request related to a problem? Please describe

With #3834 enums are now required to be in all caps, and seem to be treated equal to normal constants. However, existing code bases can't simply change all enum values, as this backwards incompatible, and adding an ignore to each member is a lot of noise.

Describe the solution you'd like

Add a new regex or option to change the allowed way enum values can look

[cdce8p]

I agree. Adding a new naming style might be the way to go. Maybe something like --class-const-naming-style. This could also be applied to class attributes annotated with Final. @Pierre-Sassoulas What do you think?

http://pylint.pycqa.org/en/latest/user_guide/options.html#predefined-naming-styles

[Pierre-Sassoulas]

Makes sense !

[cdce8p]

Opened #4185 to add --class-const-naming-style