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.

Changed compatibility logic to support a potential Python 4.0 release.

Fixes several misspellings, typos and styling errors (e.g. github ->
GitHub) in the comments and documentation.

ParserError is a subclass of ValueError, so this just makes the tests
stricter, and this can be considered a backwards-compatible change.

Ensures that ValueErrors raised when building the datetime from an
already-parsed string will be raised as ParserError as well.

Co-authored-by: Brock Mendel <jbrockmendel@gmail.com>

For all of these tests, the most important part of the test is ensuring
that the correct tzinfo object was attached, so it makes sense to add a
convenience function that asserts that they are identical *and* have the
same tzinfo object.