Fix linting for Python3 and fix all lint issues

[mkistler]

This PR fixes the linting script so that it will run with Python3 -- now the only version supported by the core. To make the set of changes manageable, I disabled a few of the less serious lint rules. Most of the changes were to conform to these linting rules:

• Line too long
• standard import should be placed before third-party import
• third-party import should be placed before package import
• Class 'BaseService' inherits from object, can be safely removed from bases in python3
• Redefining built-in 'tuple'
• Method could be a function
• Consider using a dictionary comprehension

I also took this opportunity to add external methods for convert_model, convert_list, and encode_path_vars. so that users of the core can use these methods rather than the corresponding methods that start with "_".

[codecov[bot]]

Codecov Report

Merging #40 into master will increase coverage by 0.08%. The diff coverage is 98.5%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   97.41%   97.49%   +0.08%     
==========================================
  Files          17       17              
  Lines         541      559      +18     
==========================================
+ Hits          527      545      +18     
  Misses         14       14

Impacted Files	Coverage Δ
ibm_cloud_sdk_core/iam_token_manager.py	100% <ø> (ø)	:arrow_up:
ibm_cloud_sdk_core/cp4d_token_manager.py	100% <ø> (ø)	:arrow_up:
ibm_cloud_sdk_core/utils.py	97.82% <100%> (+0.35%)	:arrow_up:
...oud_sdk_core/authenticators/basic_authenticator.py	100% <100%> (ø)	:arrow_up:
ibm_cloud_sdk_core/get_authenticator.py	100% <100%> (ø)	:arrow_up:
...loud_sdk_core/authenticators/cp4d_authenticator.py	100% <100%> (ø)	:arrow_up:
..._core/authenticators/bearer_token_authenticator.py	100% <100%> (ø)	:arrow_up:
ibm_cloud_sdk_core/api_exception.py	100% <100%> (ø)	:arrow_up:
ibm_cloud_sdk_core/jwt_token_manager.py	100% <100%> (+2.08%)	:arrow_up:
ibm_cloud_sdk_core/detailed_response.py	100% <100%> (ø)	:arrow_up:
... and 4 more

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[mkistler]

I will fix the coverage so that the PR checks pass. We should probably add pylint to PR checks as well.

[mkistler]

@mgrattray I decided to back off on raising the ValueError until the next major release, just for safety.

[padamstx]

:tada: This PR is included in version 1.1.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: