Midtransclient

Main changes:
- Fix bug custom header

Main changes:
- add Core API Subscription feature implementation
- add Core API Gopay Tokenization feature implementation

**Full Changelog**: https://github.com/Midtrans/midtrans-python-client/compare/v1.3.0...v1.4.0

Main changes:
- drop support for Python 2 (because Python 2 has reached its end of life), in favor of:
- improve compatibility with Python 3
- prevent package fail to be properly installed on Windows OS env
- update setup.py by following official pypa sample

- Latest stable version that support Python 2 (and also Python 3)
- Please check [Readme](https://github.com/Midtrans/midtrans-python-client/blob/v1.2.0/README.md) or https://pypi.org/pypi/midtransclient/1.2.0/ for feature description