Allow Observer to be used as a context manager (fixes #1090)

[Prosperibe12]

Fixes #1090

Adds context manager support to Observer to ensure
correct startup and teardown semantics.

Eg 1
class MyEventHandler(FileSystemEventHandler):
def on_any_event(self, event: FileSystemEvent) -> None:
print(event)

event_handler = MyEventHandler()
observer = Observer()
observer.schedule(event_handler, ".", recursive=True)

with observer:
while True:
time.sleep(1)

Eg 2
if name == "main":
logging.basicConfig(level=logging.INFO,
format='%(asctime)s - %(message)s',
datefmt='%Y-%m-%d %H:%M:%S')
path = sys.argv[1] if len(sys.argv) > 1 else '.'
event_handler = LoggingEventHandler()
observer = Observer()
observer.schedule(event_handler, path, recursive=True)
observer.start()

with observer:
    while True:
        time.sleep(1)

[Prosperibe12]

@BoboTiG kindly review the PR to the proposed improvement

[BoboTiG]

Thanks @Prosperibe12 !

That seems good for a first quick look.

Can you add a line in the changelog and fix tests + linters?

[Prosperibe12]

@BoboTiG I have made few changes to fix the tests, linters and updated the changelog.

[BoboTiG]

Suggested change

	- [core] Add context manager support to ``Observer`` class. The observer can now be used with a ``with`` statement for automatic start/stop management. (`#XXXX <https://github.com/gorakhargosh/watchdog/pull/1149>`__)
	- [core] Add context manager support to ``Observer`` class. The observer can now be used with a ``with`` statement for automatic start/stop management. (`#1090 <https://github.com/gorakhargosh/watchdog/pull/1149>`__)

[BoboTiG]

You can revert this change.

[BoboTiG]

This is a little bit hard to review.

I would like a PR with only Observer-related changes, without changes in tests except for the new one(s) testing the context manager (do not alter other tests, this is not the purpose of such a PR, even if they are flaky). Can you do that?

[Prosperibe12]

ok

[Prosperibe12]

@BoboTiG
reverted back to only observer related changes. some tests are failing, but they also fail on master branch, these appear to be pre-existing flaky tests unrelated to my changes. The new context manager tests in tests/test_observer.py
all pass consistently.