fix: allow None for exc_tb in Traveller.__exit__ annotations (#974)

CPython calls __exit__(None, None, None) when a with block exits without an
exception, so all three parameters must accept None. exc_type and exc_val were
already optional on the travellers, but exc_tb was annotated TracebackType
without | None, so a type checker infers the traveller is not a valid context
manager and reports invalid-context-manager on with pendulum.travel_to(...).

Annotate exc_tb as TracebackType | None in BaseTraveller.__exit__ and the
time_machine-backed Traveller.__exit__ (the other Traveller subclasses inherit
from BaseTraveller).

Closes #973

Author: madhavcodez

src/pendulum/testing/traveller.py

@@ -46,7 +46,7 @@ def __exit__(
         self,
         exc_type: type[BaseException] | None,
         exc_val: BaseException | None,
-        exc_tb: TracebackType,
+        exc_tb: TracebackType | None,
     ) -> None: ...
 
     def _not_implemented(self) -> NotImplementedError:
@@ -149,7 +149,7 @@ def __exit__(
                 self,
                 exc_type: type[BaseException] | None,
                 exc_val: BaseException | None,
-                exc_tb: TracebackType,
+                exc_tb: TracebackType | None,
             ) -> None:
                 self.travel_back()