make pyflakes a bit more silent

- explictely import json, since this is used ( only to silence pyflakes)
- remove shutil as it is unused
remove unused variable (pyflakes)
fix incorrect send_data call. there is no client variable, and send_data
is a class method. Pyflakes complained about it.
Merge pull request #4780 from mscherer/fix_accelerate_various

Fix various warning on accelerate module, found by pyflakes