Cargo deny for GPL compatibility

.drone.yml

@@ -6,5 +6,7 @@ steps:
   image: rust:1.76-slim
   commands:
   - rustup component add rustfmt
+  - cargo install --locked cargo deny
   - cargo build
   - cargo test --verbose
+  - cargo deny check licenses

deny.toml

@@ -0,0 +1,59 @@
+# Root options
+
+# The graph table configures how the dependency graph is constructed and thus
+# which crates the checks are performed against
+[graph]
+all-features = false
+no-default-features = false
+
+[output]
+feature-depth = 1
+
+[licenses]
+version = 2
+unused-allowed-license = "allow"
+confidence-threshold = 0.95
+allow = [
+      "GPL-1.0",
+      "GPL-2.0",
+      "GPL-3.0",
+      "LGPL-2.0",
+      "LGPL-2.1",
+      "LGPL-3.0",
+      "AGPL-3.0",
+      "EUPL-1.2",
+      "MIT",
+      "Apache-2.0",
+      "MPL-2.0",
+      "ISC",
+      "Unicode-DFS-2016",
+      "OpenSSL",
+      "BSD-3-Clause"
+]
+
+# Some crates don't have (easily) machine readable licensing information,
+# adding a clarification entry for it allows you to manually specify the
+# licensing information
+[[licenses.clarify]]
+crate = "ring"
+expression = "MIT AND ISC AND OpenSSL"
+license-files = [
+  { path = "LICENSE", hash = 0xbd0eed23 }
+]
+
+[[licenses.clarify]]
+crate = "webpki"
+expression = "ISC"
+license-files = [
+    { path = "LICENSE", hash = 0x001c7e6c },
+]
+
+# This section is considered when running `cargo deny check bans`.
+# More documentation about the 'bans' section can be found here:
+# https://embarkstudios.github.io/cargo-deny/checks/bans/cfg.html
+[bans]
+multiple-versions = "warn"
+wildcards = "allow"
+highlight = "all"
+workspace-default-features = "allow"
+external-default-features = "allow"