Cargo.toml: sort dependencies by type

In particular, distinguish optional crates between those
that are necessary for some named feature (e.g. macros),
and those that can be activated as their own feature
(e.g. anyhow).

Cargo.toml

@@ -15,23 +15,29 @@ edition = "2018"
 links = "python"
 
 [dependencies]
-cfg-if = { version = "1.0" }
-eyre = {version = ">= 0.4, < 0.7" , optional = true}
-# indoc must stay at 0.3.x for Rust 1.41 compatibility
-indoc = { version = "0.3.6", optional = true }
-inventory = { version = "0.1.4", optional = true }
+cfg-if = "1.0"
 libc = "0.2.62"
 parking_lot = "0.11.0"
-num-bigint = { version = "0.4", optional = true }
-num-complex = { version = ">= 0.2, < 0.5", optional = true }
-# must stay at 0.1.x for Rust 1.41 compatibility
-paste = { version = "0.1.18", optional = true }
+
+# support crates for macros feature
 pyo3-macros = { path = "pyo3-macros", version = "=0.14.5", optional = true }
+# indoc must stay at 0.3.x for Rust 1.41 compatibility
+indoc = { version = "0.3.6", optional = true }
+paste = { version = "0.1.18", optional = true }
 unindent = { version = "0.1.4", optional = true }
+
+# support crate for multiple-pymethods feature
+# must stay at 0.1.x for Rust 1.41 compatibility
+inventory = { version = "0.1.4", optional = true }
+
+# crate integrations that can be added using the eponymous features
+anyhow = { version = "1.0", optional = true }
+eyre = { version = ">= 0.4, < 0.7" , optional = true }
 hashbrown = { version = ">= 0.9, < 0.12", optional = true }
 indexmap = { version = ">= 1.6, < 1.8", optional = true }
-serde = {version = "1.0", optional = true}
-anyhow = { version = "1.0", optional = true }
+num-bigint = { version = "0.4", optional = true }
+num-complex = { version = ">= 0.2, < 0.5", optional = true }
+serde = { version = "1.0", optional = true }
 
 [dev-dependencies]
 assert_approx_eq = "1.1.0"