Skip tests on python implementations without getrefcount()

PyPy is one of these.
Added test to verify copysize handling in copy_expert

Not an original psycopg2 bug but present in pure python implementation,
e.g. ctypes issue #25 and cffi issue #5.

https://github.com/mvantellingen/psycopg2-ctypes/issues/25
https://github.com/chtd/psycopg2cffi/pull/5
Added regression test for infinity parsing in dates

...and a couple of typos.