Explicitly export cron symbols for typecheckers

[spladug]

Mypy with no_implicit_reexport = true does not see the symbols in sentry_sdk.crons as exported.

my_file.py:10: error: Module "sentry_sdk.crons" does not explicitly export attribute "monitor"  [attr-defined]

Adding the X as X to the imports marks the symbol as exported and silences the error.

An alternative approach would be to list the symbols in __all__. I'm happy to rework this if you'd prefer that.

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport

I've not audited other parts of the library for this; this just fixes the one module that I'm hitting issues with.

[sentrivana]

Hey @spladug, thanks for this PR! I'd prefer we go with __all__ here -- are you still interested in reworking this?

[spladug]

Sure thing!

[spladug]

Rebased and updated to use __all__ instead, @sentrivana!

[sentrivana]

Thanks @spladug, merged. :)