Initialize logger as a child of the Kivy's one

[misl6]

Preface

Kivy's logging has been recently improved via:

• Added support for KIVY_LOG_MODE kivy#7988
• #7978: Don't monkey-patch logging.root kivy#7979
  .. and more minor fixes + tests

When Kivy is in KIVY_LOG_MODE=KIVY (which is the default), the root logger level is set to logging.root.setLevel(logging.NOTSET), leaving the sub-loggers (and the Kivy logger) the ability to be the limiting factor.

With the root logger set to NOTSET, pyjnius started to send debug messages, even if the Kivy logger is set to silence debug messages.

This is something expected for third-party libraries and KIVY_LOG_MODE=MIXED should be used instead, as explained here: https://kivy.org/doc/master/api-kivy.logger.html#mixed-mode.

Considerations

pyjnius is usable even outside of a Kivy project, but is still part of the Kivy ecosystem, and IMHO should log as Kivy does.

By using a child logger of the Kivy main logger, if the Kivy logger is set to hide debug messages, also the log messages from pyjnius will be filtered.

From https://docs.python.org/3/library/logging.html#logging.Logger.setLevel:

When a logger is created, the level is set to NOTSET (which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger). Note that the root logger is created with level WARNING.
The term ‘delegation to the parent’ means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached.
If an ancestor is found with a level other than NOTSET, then that ancestor’s level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled.
If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root’s level will be used as the effective level.

Nothing changes when pyjnius is used in a non-kivy project (except that the logger name now have the kivy prefix).

👉 In order to make it fully compatible with the Kivy logger, pyjnius will also need to follow the colon separation logic, and will be nice to have an option to "mute" the pyjnius logger, directly from the Kivy configuration. (I'm scheduling it for Kivy 2.2.0)

FYI:
Everything started from issue: kivy/python-for-android#2704

[AndreMiras]

Another nice investigation/fix thanks