Bump min version of pebble to 4.6.0

[sanjayankur31]

I think bluepyopt needs pebble>=4.6.0. With 4.3.x on Fedora, I see tests failing:

>           with pebble.ProcessPool(                                                                                                                                                         max_workers=1,                                                                                                                                                               max_tasks=1,                                                                                                                                                                 context=multiprocessing_context                                                                                                                                          ) as pool:            
E           TypeError: ProcessPool.__init__() got an unexpected keyword argument 'context'

The context argument seems to have only been added in 4.6.0:

https://github.com/noxdafox/pebble/releases/tag/4.6.0

[anilbey]

Thanks @sanjayankur31 !