Fix MyPy errors

miplearn/solvers/learning.py

@@ -184,6 +184,7 @@ class LearningSolver:
             logger.info("Solving root LP relaxation...")
             lp_stats = self.internal_solver.solve_lp(tee=tee)
             stats.update(cast(LearningSolveStats, lp_stats.__dict__))
+            assert lp_stats.lp_wallclock_time is not None
             logger.info(
                 "LP relaxation solved in %.2f seconds" % lp_stats.lp_wallclock_time
             )
@@ -250,6 +251,7 @@ class LearningSolver:
             user_cut_cb=user_cut_cb,
             lazy_cb=lazy_cb,
         )
+        assert mip_stats.mip_wallclock_time is not None
         logger.info("MIP solved in %.2f seconds" % mip_stats.mip_wallclock_time)
         stats.update(cast(LearningSolveStats, mip_stats.__dict__))
         stats["Solver"] = "default"