use 2021 edition

Author: bast

Cargo.toml

@@ -4,7 +4,7 @@ version = "2.1.0"
 authors = ["Radovan Bast <bast@users.noreply.github.com>"]
 description = "Numerical integration grid for molecules."
 license = "MPL-2.0"
-edition = "2018"
+edition = "2021"
 readme = "README.md"
 homepage = "https://github.com/dftlibs/numgrid/"