Use PEP440 style for range formatting

Author: konstin

src/range.rs

@@ -748,7 +748,7 @@ impl<V: Display + Eq> Display for Range<V> {
                     (Included(v), Unbounded) => write!(f, ">={v}")?,
                     (Included(v), Included(b)) => {
                         if v == b {
-                            write!(f, "{v}")?
+                            write!(f, "=={v}")?
                         } else {
                             write!(f, ">={v}, <={b}")?
                         }

tests/examples.rs

@@ -237,13 +237,13 @@ fn confusing_with_lots_of_holes() {
     };
     assert_eq!(
         &DefaultStringReporter::report(&derivation_tree),
-        r#"Because there is no available version for bar and foo 1 | 2 | 3 | 4 | 5 depends on bar, foo 1 | 2 | 3 | 4 | 5 is forbidden.
-And because there is no version of foo in <1 | >1, <2 | >2, <3 | >3, <4 | >4, <5 | >5 and root 1 depends on foo, root 1 is forbidden."#
+        r#"Because there is no available version for bar and foo ==1 | ==2 | ==3 | ==4 | ==5 depends on bar, foo ==1 | ==2 | ==3 | ==4 | ==5 is forbidden.
+And because there is no version of foo in <1 | >1, <2 | >2, <3 | >3, <4 | >4, <5 | >5 and root ==1 depends on foo, root ==1 is forbidden."#
     );
     derivation_tree.collapse_no_versions();
     assert_eq!(
         &DefaultStringReporter::report(&derivation_tree),
-        "Because foo depends on bar and root 1 depends on foo, root 1 is forbidden."
+        "Because foo depends on bar and root ==1 depends on foo, root ==1 is forbidden."
     );
     assert_eq!(
         derivation_tree.packages(),