bind: ensure enum.Enum is imported in generated code as needed

This fix removes the static import of enum.Enum that was added only to the exe preamble, and replaces it with a conditional import added to all generated packages, only if there is at least one enum to generate.
The conditional import improves python2 legacy support where Enum is an external dependency that must be present.

Author: justinfx

bind/gen.go

@@ -250,7 +250,6 @@ os.chdir(cwd)
 # %[2]s
 
 import _%[1]s, collections
-from enum import Enum
 
 # to use this code in your end-user python file, import it as follows:
 # from %[1]s import %[3]s
@@ -776,6 +775,10 @@ func (g *pyGen) genAll() {
 	}
 
 	g.pywrap.Printf("\n\n#---- Enums from Go (collections of consts with same type) ---\n")
+	// conditionally add Enum support because it is an external dependency in py2
+	if len(g.pkg.enums) > 0 {
+		g.pywrap.Printf("from enum import Enum\n\n")
+	}
 	for _, e := range g.pkg.enums {
 		g.genEnum(e)
 	}