Tighten up type of Deploy._process_exports

This is needed because the CovMerge constructor updates
self.exports (which is generated with that function), so it needs the
stricter dict type. We also know the key and value types, so we may as
well give them as well.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

Author: rswarbrick

src/dvsim/job/deploy.py

@@ -276,7 +276,7 @@ def _subst_vars(self, ignored_subst_vars=None) -> None:
             ignore_error=False,
         )
 
-    def _process_exports(self) -> Mapping:
+    def _process_exports(self) -> dict[str, str]:
         """Convert 'exports' as a list of dicts in the HJson to a dict.
 
         Exports is a list of key-value pairs that are to be exported to the