Exclude pyjslib.py from python3 syntax correction check

pyjs doesn't support python3 and it's unmaintained.
async is a new keyword introduced in python 3.7.

./tests/tools/check_source.sh
Syntax checking using python3 ...
Python 3.7.2+

Syntax error in ./runtime/Worker.py
File "./svgui/pyjs/lib/pyjslib.py", line 24
def import_module(path, parent_module, module_name, dynamic=1, async=False):
^
SyntaxError: invalid syntax

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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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# See COPYING.Runtime file for copyrights details.

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LogLevels = ["CRITICAL", "WARNING", "INFO", "DEBUG"]

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LogLevelsCount = len(LogLevels)

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LogLevelsDict = dict(zip(LogLevels, range(LogLevelsCount)))

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LogLevelsDefault = LogLevelsDict["DEBUG"]