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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[TOPOLOGY]

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NetName=None

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Nodes=0x01

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Node64Present=0x01

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Node64Name=micromod

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Node64DCFName=PEAK MicroMod.eds

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EDSBaseName=eds