Change a few list sorting orders in generated code

This commit combines two changes that have only a non-functional impact
on generated Python code:

  o The enumeration lists (namedValues) are sorted by numeric value
    rather than by name. The new sorting order is more in line with what
    is found in most MIBs, usually easier to read, and more consistent
    with the sorting order before the introduction of Jinja2 templates.

  o The static list of imported constraint classes is rearranged to be
    alphabetic. The previous order made little sense.

Seeing as neither aspect should be relied upon, no tests are added here.

pysmi/codegen/templates/pysnmp/mib-definitions.j2

@@ -22,16 +22,16 @@ Managed Objects Instances.
 
 {% block asn1_constraints_imports scoped %}
 (ConstraintsIntersection,
+ ConstraintsUnion,
  SingleValueConstraint,
  ValueRangeConstraint,
- ValueSizeConstraint,
- ConstraintsUnion) = mibBuilder.importSymbols(
+ ValueSizeConstraint) = mibBuilder.importSymbols(
     "ASN1-REFINEMENT",
     "ConstraintsIntersection",
+    "ConstraintsUnion",
     "SingleValueConstraint",
     "ValueRangeConstraint",
-    "ValueSizeConstraint",
-    "ConstraintsUnion")
+    "ValueSizeConstraint")
 {% endblock -%}
 
 {% block smi_imports scoped %}
@@ -127,7 +127,7 @@ if mibBuilder.loadTexts:
         )
     )
     namedValues = NamedValues(
-    {% for name, iden in spec['enumeration'].items()|sort %}
+    {% for name, iden in spec['enumeration'].items()|sort(attribute='1') %}
         {% if loop.first and loop.last %}
         ("{{ name}}", {{ iden }})
         {% elif loop.first %}
@@ -156,7 +156,7 @@ if mibBuilder.loadTexts:
 
 {% macro bits(namedbits) %}
     namedValues = NamedValues(
-    {% for name, iden in namedbits.items()|sort %}
+    {% for name, iden in namedbits.items()|sort(attribute='1') %}
         {% if loop.first and loop.last %}
         ("{{ name}}", {{ iden }})
         {% elif loop.first %}