sentry: Ignore SystemExit and similar exceptions.

There are three exceptions in Python3 which are descended from
BaseException, but not Exception: GeneratorExit, KeyboardInterrupt,
and SystemExit.  None of these are suitable to be sent to Sentry.
For example, SystemExit is raised by `sys.exit`; in that sense, it is
never "uncaught" because we chose to cause it explicitly.

Use the suggested form[1] for ignoring specific classes of exceptions.

[1] getsentry/sentry-python#149 (comment)

zproject/sentry.py

@@ -12,7 +12,12 @@
 if TYPE_CHECKING:
     from sentry_sdk._types import Event, Hint
 
-def add_context(event: 'Event', hint: 'Hint') -> 'Event':
+def add_context(event: 'Event', hint: 'Hint') -> Optional['Event']:
+    if "exc_info" in hint:
+        _, exc_value, _ = hint["exc_info"]
+        # Ignore GeneratorExit, KeyboardInterrupt, and SystemExit exceptions
+        if not isinstance(exc_value, Exception):
+            return None
     from zerver.models import get_user_profile_by_id
     with capture_internal_exceptions():
         user_info = event.get("user", {})