Problem: The python target regenerates setup.py

The only reason to regenerate setup.py is to update changes in project name, version and other project metadata.
Some bindings may need more complex setup configuration than provided by the generator, so it makes more sense to
only regenerate a separate file containing project name, version, etc. and read this file from within setup.py

Author: lbotsch

zproject_python.gsl

@@ -243,28 +243,43 @@ endfunction
 
 function generate_setup_py
     directory.create ("bindings/python")
-    output "bindings/python/setup.py"
+    output "bindings/python/_project.py"
     >$(project.GENERATED_WARNING_HEADER:)
-    >from setuptools import setup
-    >
-    >setup(
-    >    name = "$(project.name:c)",
-    >    version = "$(project->version.major).$(project->version.minor).$(project->version.patch)",
-    >    license = "$(project.license)",
-    >    description = """Python bindings of: $(project.description)""",
+    >NAME        = "$(project.name:c)"
+    >VERSION     = "$(project->version.major).$(project->version.minor).$(project->version.patch)"
+    >LICENSE     = "$(project.license)"
+    >DESCRIPTION = """Python bindings of: $(project.description)"""
     if defined (project.repository)
-    >    url = "$(project.repository)",
+    >URL         = "$(project.repository)"
+    else
+    >URL         = ""
     endif
-    >    packages = ["$(project.name:c)"],
-    >    install_requires = [
-   for project.use
-       if count (project->dependencies.class, class.project = use.project) > 0
-    >        "$(use.project)",
-       endif
-   endfor
-    >    ],
-    >)
+    >PACKAGES    = ["$(project.name:c)"]
+    >REQUIRES    = [
+    for project.use
+    if count (project->dependencies.class, class.project = use.project) > 0
+    >    "$(use.project)",
+    endif
+    endfor
+    >]
     >$(project.GENERATED_WARNING_HEADER:)
+
+    if !file.exists ("bindings/python/setup.py")
+        output "bindings/python/setup.py"
+        >import os.path
+        >from _project import NAME, VERSION, LICENSE, DESCRIPTION, URL, PACKAGES, REQUIRES
+        >from setuptools import setup
+        >
+        >setup(
+        >    name = NAME,
+        >    version = VERSION,
+        >    license = LICENSE,
+        >    description = DESCRIPTION,
+        >    url = URL,
+        >    packages = PACKAGES,
+        >    install_requires = REQUIRES,
+        >)
+    endif
 endfunction
 
 function generate_binding