Improve the way we make the subplot tests opt-in.

- Promote subplotlib to a proper dependency, as dev-dependencies can
    not be optional.

Cargo.toml

@@ -53,6 +53,7 @@ tokio = { version = "1.13.1" }
 rpassword = "7.0"
 serde_json = "1.0.80"
 serde = { version = "1.0.137", features = ["derive"] }
+subplotlib = { version = ">=0.7, <0.10", optional = true }
 terminal_size = ">=0.2.6, <0.4"
 termcolor = "1.2.0"
 textwrap = ">=0.15, <0.17"
@@ -75,7 +76,6 @@ cfg-if = "1"
 terminal_size = ">=0.2.6, <0.4"
 
 [dev-dependencies]
-subplotlib = ">=0.7, <0.10"
 fehler = "1.0.0"
 assert_cmd = "2"
 predicates = ">=2, <4"
@@ -87,6 +87,11 @@ name = "sq"
 path = "src/main.rs"
 bench = false
 
+[[test]]
+name = "sq-subplot"
+path = "tests/sq-subplot.rs"
+required-features = ["subplot"]
+
 [features]
 default = [
     "crypto-nettle",
@@ -97,7 +102,7 @@ crypto-botan = ["sequoia-openpgp/crypto-botan"]
 crypto-botan2 = ["sequoia-openpgp/crypto-botan2"]
 crypto-cng = ["sequoia-openpgp/crypto-cng"]
 crypto-rust = ["sequoia-openpgp/crypto-rust"]
-subplot = ["subplot-build"]
+subplot = ["subplot-build", "subplotlib"]
 
 [profile.release]
 debug = true

tests/sq-subplot.rs

@@ -1,2 +1 @@
-#![cfg(feature = "subplot")]
 include!(concat!(env!("OUT_DIR"), "/sq.rs"));