Fix TemplateSyntaxError pickling

[andrewrabert]

Implement __reduce__ to fix pickling of TemplateSyntaxError

New test output without the fix (Python 3.8):

________________________________________ TestDebug.test_pickleable_syntax_error _________________________________________

self = <test_debug.TestDebug object at 0x109740490>, fs_env = <jinja2.environment.Environment object at 0x1097400a0>

    def test_pickleable_syntax_error(self, fs_env):
        exc = TemplateSyntaxError('wtf', 42)
        pickled_exc = pickle.dumps(exc)
>       unpickled_exc = pickle.loads(pickled_exc)
E       TypeError: __init__() missing 1 required positional argument: 'lineno'

test_debug.py:78: TypeError

[davidism]

Interesting. Seems like it would be better to address why lineno isn't getting pickled/unpickled.

[andrewrabert]

Some findings:

• https://bugs.python.org/issue32696
• https://bugs.python.org/issue1692335#msg310951

[andrewrabert]

@davidism Changed the fix to only override __reduce__. No other changes to the exception.