Update clap requirement from ~3.0.0 to ~3.1.3 (#215)

Author: dependabot[bot]

Cargo.toml

@@ -28,7 +28,7 @@ tskit-derive = {version = "0.2.0", path = "tskit-derive", optional = true}
 mbox = "0.6.0"
 
 [dev-dependencies]
-clap = "~3.0.0"
+clap = "~3.1.3"
 serde = {version = "1.0.118", features = ["derive"]}
 serde-pickle = "1.1.0"
 bincode = "1.3.1"

examples/forward_simulation.rs

@@ -18,7 +18,7 @@
  *   and numbers of crossovers, etc.., from being entered
  *   on the command line.
  */
-use clap::{App, Arg};
+use clap::{Command, Arg};
 use rand::rngs::StdRng;
 use rand::Rng;
 use rand::SeedableRng;
@@ -65,7 +65,7 @@ impl SimParams {
     fn new() -> Self {
         let mut params = SimParams::default();
 
-        let matches = App::new("forward_simulation")
+        let matches = Command::new("forward_simulation")
             .arg(
                 Arg::new("popsize")
                     .short('N')

examples/tree_traversals.rs

@@ -1,4 +1,4 @@
-use clap::{App, Arg};
+use clap::{Command, Arg};
 use tskit::prelude::*;
 
 // "Manual" traversal from samples to root
@@ -25,7 +25,7 @@ fn preorder_traversal(tree: &tskit::Tree) {
 }
 
 fn main() {
-    let matches = App::new("tree_traversals")
+    let matches = Command::new("tree_traversals")
         .arg(
             Arg::new("treefile")
                 .short('t')