Import collections.abc instead of collections

[lurunming]

Importing ABCs from collectiions is deprecated since Python 3.3 and it
will stop working in 3.9. Use import collections.abc instead

[google-cla]

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[lurunming]

@googlebot I signed it!

[mina86]

This is not the repository you’re looking for. https://github.com/mina86/pygtrie has this implemented.