Set pyproject=true in buildPythonApplication

The buildPythonPackage and buildPythonApplication functions now
  require an explicit format attribute. Previously the default format
  used setuptools and called setup.py from the source tree, which is
  deprecated. The modern alternative is to configure pyproject = true
  with build-system = [ setuptools ].

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/meteocat-exporter/default.nix

@@ -1,9 +1,11 @@
 { python3Packages, lib }:
 
-python3Packages.buildPythonApplication rec {
+python3Packages.buildPythonApplication {
   pname = "meteocat-exporter";
   version = "1.0";
 
+  pyproject = true;
+
   src = ./.;
 
   doCheck = false;