fix(tracing) : Add trace to __all__ in top-level __init__.py

[lobsterkatie]

Currently, using the decorator form of trace like this (as mentioned in the docs here):

import sentry_sdk

@sentry_sdk.trace
def do_stuff():

causes mypy to throw a Module "sentry_sdk" does not explicitly export attribute "trace" [attr-defined] error. This adds trace to the top-level __init__.py's __all__ so mypy sees it as being officially exported and stops throwing the error.