increase 'serde' version to 1.0.103, the earliest version that actually works

You can now run	`cargo +nightly update -Z minimal-versions` and then
actually build this library.

Author: apoelstra

Cargo.toml

@@ -27,7 +27,7 @@ bitcoin = { version = "0.29.1", default-features = false }
 hashbrown = { version = "0.11", optional = true }
 
 # Do NOT use this as a feature! Use the `serde` feature instead.
-actual-serde = { package = "serde", version = "1.0", optional = true }
+actual-serde = { package = "serde", version = "1.0.103", optional = true }
 
 [dev-dependencies]
 serde_test = "1.0.147"
@@ -65,4 +65,4 @@ required-features = ["compiler","std"]
 
 [[example]]
 name = "psbt_sign_finalize"
-required-features = ["std", "base64"]
+required-features = ["std", "base64"]