Don't import psycopg2.tz into the C extension

This makes possible to import _psycopg directly, after adding the
package directory to the pythonpath. This enables hacks such as:

    sys.path.insert(0, '/path/to/psycopg2')
    import _psycopg
    sys.modules['psycopg2._psycopg'] = _psycopg
    sys.path.pop(0)

which can work around e.g. the problem of #201, freeze that cannot
freeze psycopg2. Well, freeze cannot freeze it because it's just not
designed to deal with C extensions. At least now the frozen application
can hack the pythonpath and work around the limitation by importing
_psycopg as above and then doing the rest of the imports normally.

Keeping long-lived references to python objects is bad anyway: the
tz module couldn't be reloaded before.

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