avocado/core/tree.py: properly copy the TreeEnvironment

On Python 3, trying to copy a FilterSet, using the instance.copy()
method, will return a plain set() object.  This influences a number of
code, including the fingerprint generation, and the variant ID which
will be based on the hash of the fingerprint.

Let's make things predictable and copy using Python's standard module
copy.

This fixes the functional test in module `test_getdata` under Python
3, as the variant IDs are now the same on jobs run under Python 2 and
3.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/tree.py

@@ -34,6 +34,7 @@ original base tree code and re-license under GPLv2+, given that GPLv3 and GPLv2
 """
 
 import collections
+import copy
 import itertools
 import locale
 
@@ -74,9 +75,9 @@ class TreeEnvironment(dict):
     def copy(self):
         cpy = TreeEnvironment()
         cpy.update(self)
-        cpy.origin = self.origin.copy()
-        cpy.filter_only = self.filter_only.copy()
-        cpy.filter_out = self.filter_out.copy()
+        cpy.origin = copy.copy(self.origin)
+        cpy.filter_only = copy.copy(self.filter_only)
+        cpy.filter_out = copy.copy(self.filter_out)
         return cpy
 
     def __str__(self):