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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<?xml version='1.0' encoding='utf-8'?>

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<BeremizRoot URI_location="LOCAL://">

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  <TargetType/>

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  <Libraries Enable_SVGUI_Library="true" Enable_Python_Library="true"/>

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</BeremizRoot>