Fix running single spend Python test module
Fix running single spend Python test module

i.e. fix `make python-single-uat TEST_NAME=test_spend_tx.py`.

Symptom:
```
...
Swagger python tempdir: /tmp/tmp.40U8EEqc2Q
( cd py && TEST_NAME=test_spend_tx.py make single-uat; )
make[1]: Entering directory '/home/builder/epoch/py'
( cd /home/builder/epoch/py/tests && /home/builder/epoch/py/bin/nosetests  --nocapture -c /home/builder/epoch/py/tests/nose.cfg --tc-file /home/builder/epoch/py/tests/integration/setup.yaml --tc-format yaml /home/builder/epoch/py/tests/integration/test_spend_tx.py; )
nose.config: INFO: Ignoring files matching ['^\\.', '^_', '^setup\\.py$']
integration.test_spend_tx.test_not_enough_tokens ... ERROR

======================================================================
ERROR: integration.test_spend_tx.test_not_enough_tokens
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/builder/epoch/py/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/builder/epoch/py/tests/integration/test_spend_tx.py", line 30, in test_not_enough_tokens
    bob = common.setup_beneficiary()
  File "/home/builder/epoch/py/tests/integration/common.py", line 288, in setup_beneficiary
    import keys
ImportError: No module named keys
```

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch