logging.py: CPython-compatible logging improvements.

[ned-pcs]

This PR allows for logging handlers to be added to the root logger and then used by non-root loggers that don't have their own handlers.

It replaces #750 (in which I forgot a period at the end of a commit message title).

It also adds the (CPython-compatible) handlers argument to logging.basicConfig() to ease this initialization:

    import logging
    sh = logging.StreamHandler()
    fh = logging.FileHandler("my.log", mode="a")
    logging.basicConfig(handlers=[sh, fh])

    root_logger = logging.getLogger() # uses sh and fh
    another_logger = logging.getLogger("another") # inherits handlers

It also adds the Logger.removeHandler() method and avoids repeated handler addition.

It also adds the flush() method to StreamHandler and its subclasses.

It also correctly calls the superclass constructor from the StreamHandler constructor and uses a default formatter if a Handler has none set (as in PR #710).

[ned-pcs]

build failure is due to an unrelated manifest.py issue.

[projectgus]

@ned-pcs If you rebase on master then the build should be fixed now.

[dpgeorge]

Should there still be a close() method which just calls self.flush()?

[ned-pcs]

Good question. According to the Cpython docs, Cpython does not have a StreamHandler.close() method, and so the inherited close() method doesn't do any output. I originally had a close() method that called flush() but removed it after looking at the Cpython documentation.