Imports BaseCredentials error

[drewxa]

Hi. The file imports BaseCredentials, but this class was removed in the latest release. As a result, there are unit test errors in the code. The error messages are as follows:

FAIL:   File "python-amazon-ad-api/ad_api/api/dsp/__init__.py", line 1, in <module>
FAIL:     from .reports import Reports
FAIL:   File "python-amazon-ad-api/ad_api/api/dsp/reports.py", line 1, in <module>
FAIL:     from ad_api.api.dsp.client import DspClient as Client
FAIL:   File "python-amazon-ad-api/ad_api/api/dsp/client.py", line 2, in <module>
FAIL:     from ad_api.auth.credentials import BaseCredentials
FAIL: ImportError: cannot import name 'BaseCredentials' from 'ad_api.auth.credentials' (python-amazon-ad-api/ad_api/auth/credentials.py)
FAIL: ad_api.api.dsp.access_token_client cannot import name 'BaseCredentials' from 'ad_api.auth.credentials' (python-amazon-ad-api/ad_api/auth/credentials.py)

I recon to resolve this issue, it needs to update the code in client.py and any other modules that use BaseCredentials.

[denisneuf]

#145