Convert Python2 to Python3 for client.py.

[xcollantes]

Run python-modernize on src/gdata/client.py and deleted backup file produced by conversion.

xcollantes@

[googlebot]

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[xcollantes]

@googlebot I signed it! I didn't actually but according to go/github-tvc, I shouldn't sign it.
xcollantes@google.com

[xcollantes]

I have signed the CLA.