Fix check for Starlette in FastAPI integration (#1868)

When loading FastAPI integration also check if StarletteIntegration can actually be loaded. (Because Starlette is a requirement for FastAPI)

Fixes #1603

Author: antonpirker

sentry_sdk/integrations/fastapi.py

@@ -3,18 +3,21 @@
 from sentry_sdk._types import MYPY
 from sentry_sdk.hub import Hub, _should_send_default_pii
 from sentry_sdk.integrations import DidNotEnable
-from sentry_sdk.integrations.starlette import (
-    StarletteIntegration,
-    StarletteRequestExtractor,
-)
 from sentry_sdk.tracing import SOURCE_FOR_STYLE, TRANSACTION_SOURCE_ROUTE
 from sentry_sdk.utils import transaction_from_function
 
 if MYPY:
     from typing import Any, Callable, Dict
-
     from sentry_sdk.scope import Scope
 
+try:
+    from sentry_sdk.integrations.starlette import (
+        StarletteIntegration,
+        StarletteRequestExtractor,
+    )
+except DidNotEnable:
+    raise DidNotEnable("Starlette is not installed")
+
 try:
     import fastapi  # type: ignore
 except ImportError: