bpo-34002: Minor efficiency and clarity improvements in email package. (GH-7999)

* Check intersection of two sets explicitly

Comparing ``len(a) > ``len(a - b)`` is essentially looking for an
intersection between the two sets. If set ``b`` does not intersect ``a``
then ``len(a - b)`` will be equal to ``len(a)``. This logic is more
clearly expressed as ``a & b``.

* Change while/pop to a for-loop

Copying the list, then repeatedly popping the first element was
unnecessarily slow. I also cleaned up a couple other inefficiencies.
There's no need to unpack a tuple, then re-pack and append it. The list
can be created with the first element instead of empty. Secondly, the
``endswith`` method returns a bool, so there's no need for an if-
statement to set ``encoding`` to True or False.

* Use set.intersection to check for intersections

``a.intersection(b)`` method is more clear of purpose than ``not
a.isdisjoint(b)`` and avoids an unnecessary set construction that ``a &
set(b)`` performs.

* Use not isdisjoint instead of intersection

While it reads slightly worse, the isdisjoint method will stop when it
finds a counterexample and returns a bool, rather than looping over the
entire iterable and constructing a new set.

Lib/email/utils.py

@@ -259,21 +259,13 @@ def decode_params(params):
 
     params is a sequence of 2-tuples containing (param name, string value).
     """
-    # Copy params so we don't mess with the original
-    params = params[:]
-    new_params = []
+    new_params = [params[0]]
     # Map parameter's name to a list of continuations.  The values are a
     # 3-tuple of the continuation number, the string value, and a flag
     # specifying whether a particular segment is %-encoded.
     rfc2231_params = {}
-    name, value = params.pop(0)
-    new_params.append((name, value))
-    while params:
-        name, value = params.pop(0)
-        if name.endswith('*'):
-            encoded = True
-        else:
-            encoded = False
+    for name, value in params[1:]:
+        encoded = name.endswith('*')
         value = unquote(value)
         mo = rfc2231_continuation.match(name)
         if mo: