Pin `serde` to 1.0.142

1.0.143 bumps MSRV higher than what we support.

contrib/test.sh

@@ -24,6 +24,9 @@ fi
 
 # We should not have any duplicate dependencies. This catches mistakes made upgrading dependencies
 # in one crate and not in another (e.g. upgrade bitcoin_hashes in bitcoin but not in secp).
+cargo update -p serde --precise 1.0.142
+cargo update -p serde_test --precise 1.0.142
+cargo update -p serde_derive --precise 1.0.142
 duplicate_dependencies=$(cargo tree  --target=all --all-features --duplicates | wc -l)
 if [ "$duplicate_dependencies" -ne 0 ]; then
     echo "Dependency tree is broken, contains duplicates"
@@ -118,6 +121,8 @@ then
     cargo new dep_test
     cd dep_test
     echo 'bitcoin = { path = "..", features = ["serde"] }' >> Cargo.toml
+    cargo update -p serde --precise 1.0.142
+    cargo update -p serde_derive --precise 1.0.142
 
     cargo test --verbose
 fi