interpreter: Add a way to copy and environment object

It is really usefull when you have common variables defined
for all tests and then need to set some extra ones for each test

mesonbuild/interpreter.py

@@ -27,6 +27,7 @@ import os, sys, subprocess, shutil, uuid, re
 from functools import wraps
 
 import importlib
+import copy
 
 run_depr_printed = False
 
@@ -188,6 +189,7 @@ class EnvironmentVariablesHolder(InterpreterObject):
         self.methods.update({'set': self.set_method,
                              'append': self.append_method,
                              'prepend' : self.prepend_method,
+                             'copy' : self.copy_method,
                             })
 
     @stringArgs
@@ -210,6 +212,9 @@ class EnvironmentVariablesHolder(InterpreterObject):
     def prepend_method(self, args, kwargs):
         self.add_var(self.held_object.prepend, args, kwargs)
 
+    def copy_method(self, args, kwargs):
+        return copy.deepcopy(self)
+
 
 class ConfigurationDataHolder(InterpreterObject):
     def __init__(self):