python3Packages.msgspec.optional-dependencies: fix the eval

Without the change the eval fails on `master` as:

    $ nix-instantiate -A python3Packages.msgspec.optional-dependencies
    error:
       … while evaluating the attribute 'dev'
         at pkgs/development/python-modules/msgspec/default.nix:45:5:
           44|   optional-dependencies = {
           45|     dev = [
             |     ^
           46|       coverage

       … while evaluating the attribute 'test'
         at pkgs/development/python-modules/msgspec/default.nix:60:5:
           59|     ];
           60|     test = [
             |     ^
           61|       attrs

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: expected a string but found a float: 3.11