Clear the copy of the globs dict after running examples. This helps to

break cycles, which are a special problem when running generator tests
that provoke exceptions by invoking the .next() method of a named
generator-iterator:  then the iterator is named in globs, and the
iterator's frame gets a tracekback object pointing back to globs, and
gc doesn't chase these types so the cycle leaks.

Also changed _run_examples() to make a copy of globs itself, so its
callers (direct and indirect) don't have to (and changed the callers
to stop making their own copies); *that* much is a change I've been
meaning to make for a long time (it's more robust the new way).

Here's a way to provoke the symptom without doctest; it leaks at a
prodigious rate; if the last two "source" lines are replaced with
    g().next()
the iterator isn't named and then there's no leak:

source = """\
def g():
    yield 1/0

k = g()
k.next()
"""

code = compile(source, "<source>", "exec")

def f(globs):
    try:
        exec code in globs
    except ZeroDivisionError:
        pass

while 1:
    f(globals().copy())

After this change, running test_generators in an infinite loop still leaks,
but reduced from a flood to a trickle.

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