Add query retry to PyCellBase v5

[julie-sullivan]

This feature retries a query on timeout. After a wait period, retries N times.

• Already merged this change into CellBase 4.x, published in GeL's PyPi
• Not backwards compatible with Python 2.7. At all.
• I updated the version to be v5.0.0
• Didn't test the actual timeout and retry, not sure how to do that. For the 4.x version, GeL tested for me as they were experiencing a lot of CellBase timeouts.
• Default URL doesn't exist anymore (see below)
• Tests fail (see below)

[julie-sullivan]

This URL doesn't exist anymore.

## These are the RESTful configurations parameters
rest:
  hosts:
  - "bioinfodev.hpc.cam.ac.uk/cellbase-5.0.0"
  timeout: 2000

[julie-sullivan]

These tests fail without my change:

======================================================================
FAIL: test_get_clinical (tests.test_cbrestclients.GenomicRegionTest)
Checks retrieval of clinical data
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/julie/git/cellbase/cellbase-client/src/main/python/tests/test_cbrestclients.py", line 146, in test_get_clinical
    assert len(res[0]['result']) == 469
AssertionError

======================================================================
FAIL: test_get_substitution_scores (tests.test_cbrestclients.ProteinClientTest)
Checks retrieval of protein substitution scores
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/julie/git/cellbase/cellbase-client/src/main/python/tests/test_cbrestclients.py", line 76, in test_get_substitution_scores
    assert len(res[0]['result']) == 1
AssertionError

----------------------------------------------------------------------
Ran 41 tests in 194.136s