Pin some dependencies for MSRV compatibilty. (#398)

Author: sunfishcode

.github/workflows/main.yml

@@ -373,7 +373,10 @@ jobs:
         toolchain: ${{ matrix.rust }}
 
     - name: Use specific dependency versions for Rust 1.63 compatibility.
-      run: cargo update --package=once_cell --precise=1.20.3
+      run: |
+        cargo update --package=once_cell --precise=1.20.3
+        cargo update --package=async-io --precise=2.5.0
+        cargo update --package=polling --precise=3.10.0
 
     # Don't use --all-features because some of the features have dependencies
     # that don't work on newer Rust versions.