Py3.7 warnings

[pschelle]

When running pytest the latest version with Python 3.7, 2 warnings are displayed.
This PR solves them and adds Python 3.7 to travis and setup.py.

Below are the results of pytest before and after the changes.
BEFORE
=============================== test session starts ================================
platform linux -- Python 3.7.3, pytest-4.6.2, py-1.8.0, pluggy-0.12.0
rootdir: /home/peter/proj/minio-py_
collected 131 items

tests/unit/bucket_exist_test.py .... [ 3%]
tests/unit/copy_object_test.py ....... [ 8%]
tests/unit/generate_xml_test.py .. [ 9%]
tests/unit/get_bucket_policy_test.py .. [ 11%]
tests/unit/get_object_test.py ... [ 13%]
tests/unit/get_s3_endpoint_test.py .. [ 15%]
tests/unit/header_value_test.py ....... [ 20%]
tests/unit/list_buckets_test.py .. [ 22%]
tests/unit/list_incomplete_uploads_test.py ... [ 24%]
tests/unit/list_objects_test.py ... [ 26%]
tests/unit/list_objects_v2_test.py .. [ 28%]
tests/unit/list_uploaded_parts_test.py ... [ 30%]
tests/unit/make_bucket_test.py .... [ 33%]
tests/unit/minio_test.py .................. [ 47%]
tests/unit/optimal_part_test.py ....... [ 52%]
tests/unit/presigned_get_object_test.py .... [ 55%]
tests/unit/presigned_put_object_test.py ... [ 58%]
tests/unit/put_object_test.py .... [ 61%]
tests/unit/remove_bucket_test.py .... [ 64%]
tests/unit/remove_object_test.py .... [ 67%]
tests/unit/remove_objects_test.py ....... [ 72%]
tests/unit/set_bucket_notification_test.py .................. [ 86%]
tests/unit/sign_test.py ............. [ 96%]
tests/unit/stat_object_test.py .... [ 99%]
tests/unit/trace_test.py . [100%]

================================= warnings summary =================================
minio/helpers.py:56
/home/peter/proj/minio-py_/minio/helpers.py:56: DeprecationWarning: invalid escape sequence _
'^((?!-)(?!)[A-Z_\d-]{1,63}(?<!-)(?<!)\.)*((?!)(?!-)[A-Z_\d-]{1,63}(?<!-)(?<!))$',

minio/copy_conditions.py:39
/home/peter/proj/minio-py_/minio/copy_conditions.py:39: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
class CopyConditions(collections.MutableMapping):

-- Docs: https://docs.pytest.org/en/latest/warnings.html
====================== 131 passed, 2 warnings in 0.59 seconds ======================

AFTER
=============================== test session starts ================================
platform linux -- Python 3.7.3, pytest-4.6.2, py-1.8.0, pluggy-0.12.0
rootdir: /home/peter/proj/minio-py
collected 131 items

tests/unit/bucket_exist_test.py .... [ 3%]
tests/unit/copy_object_test.py ....... [ 8%]
tests/unit/generate_xml_test.py .. [ 9%]
tests/unit/get_bucket_policy_test.py .. [ 11%]
tests/unit/get_object_test.py ... [ 13%]
tests/unit/get_s3_endpoint_test.py .. [ 15%]
tests/unit/header_value_test.py ....... [ 20%]
tests/unit/list_buckets_test.py .. [ 22%]
tests/unit/list_incomplete_uploads_test.py ... [ 24%]
tests/unit/list_objects_test.py ... [ 26%]
tests/unit/list_objects_v2_test.py .. [ 28%]
tests/unit/list_uploaded_parts_test.py ... [ 30%]
tests/unit/make_bucket_test.py .... [ 33%]
tests/unit/minio_test.py .................. [ 47%]
tests/unit/optimal_part_test.py ....... [ 52%]
tests/unit/presigned_get_object_test.py .... [ 55%]
tests/unit/presigned_put_object_test.py ... [ 58%]
tests/unit/put_object_test.py .... [ 61%]
tests/unit/remove_bucket_test.py .... [ 64%]
tests/unit/remove_object_test.py .... [ 67%]
tests/unit/remove_objects_test.py ....... [ 72%]
tests/unit/set_bucket_notification_test.py .................. [ 86%]
tests/unit/sign_test.py ............. [ 96%]
tests/unit/stat_object_test.py .... [ 99%]
tests/unit/trace_test.py . [100%]

============================ 131 passed in 0.66 seconds ============================

[vadmeste]

LGTM

[pschelle]

Oops, looks like .travis.yml needs some more attention.

https://docs.travis-ci.com/user/languages/python/
see Python 3.7 and higher #You’ll need to add dist: xenial to your .travis.yml file to use Python 3.7 and higher.

[Praveenrajmani]

LGTM

[vadmeste]

LGTM