Issue 3726: Allow ',<whitespace>' delimiters in logging.config.fileConfig()

Currently, logging.config.fileConfig() inconsistently handles lines like:

[handlers]
keys = spam, eggs

[formatters]
keys = foo, bar

It does, however, correctly handle the ', ' delimiter in the [loggers]
section. This is because the various _install_*() functions in
logging.config aren't consistent about whether (and when) or not they
strip whitespace when generating key names.

Though the stdlib documentation only includes examples in the [handlers]
and [formatters] section with ',' delimiters, it seems reasonable to
expect that logging.config.fileConfig() should handle ', '. The attached
test demonstrates the failure and suggests a solution.

Thanks!
whitespace