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This was originally dead code, because an indentation error had `__repr__`
defined after the `return` statement in `__str__`. The definition of the
`__repr__` is also changed to be more in line with the conception of a
repr as "what you would have to evalue to get this object".

Even though this is not guaranteed behavior, this commit also adds a
regression test to avoid simple errors like this in the future.

In attempting to pass-through the string representation of an exception
we are wrapping, we made the erroneous assumption that `args[0]` would
always be a string (or something that can concatenate cleanly with a
string). This turns out not to be the case with `IllegalMonthError`,
where it is an integer, so to avoid raising an erroneous `TypeError`, we
first convert the wrapped exception to a string.

See GH issue #981.

Since the strong cache uses operations in OrderedDict that are not
thread-safe in Python 2, it is necessary to acquire a lock while
updating the dictionary.

When Python 2 support is dropped, we can likely refactor this to avoid
locking.

test_tzlocal_offset_equal[GMT-tzoff1] fails if a system does not
have a defined (installed) GMT timezone, for example if "Link Etc/GMT GMT"
is not defined in the zoneinfo configuration:

>           assert tz.tzlocal() == tzoff
E           AssertionError: assert tzlocal() == tzoffset(u'GMT', 0)
E             -tzlocal()
E             +tzoffset(u'GMT', 0)

An example of such a configuration is FreeBSD, who's default timezone is
UTC [1][2]. This configuration results in any "non-present" or undefined TZ
name being returned/processed as the default timezone name (in FreeBSD's
case, UTC).

Further, POSIX does not recognize [3] TZ=GMT or TZ=UTC as valid values
because they neither start with a colon nor contain a numeric offset hour
field.

Accordingly, update the GMT value for this parametrized test to GMT0, a
specific (not aliased) timezone, making it more robust to system
configuration differences.

[1] https://svnweb.freebsd.org/changeset/base/130332
[2] https://svnweb.freebsd.org/changeset/base/199405
[3] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

At the moment these tests are skipped ad-hoc in the test suite, but we
can use `pytest.skip` in the context manager itself to skip them
automatically and make things easier on ourselves.

Fix error message for gettz when passed bytes

This was originally added when we were still supporting unittest-only
tests, but now that we require pytest, we can switch over to using the
built-in pytest.warns context manager.