Added automatic generation of the __version__ attribute.

[smarie]

Fixes #253

• setuptools_scm was added as a setup dependency and ci dependency
• use_scm_version (from setuptools_scm) is used instead of version to always generate the right version number based on git.
• in addition, write_to is used in use_scm_version so that a _version.py file is automatically created when building the wheel.
• Finally the __init__.py contains code supporting the two cases: either the _version.py file is present and it should be read, or it is not present and setuptools_scm is used programmatically to generate the version based on git information

.gitignore was modified so that the _version.py file cannot be added by mistake.

All these best practices come from https://smarie.github.io/python-getversion/#package-versioning-best-practices

[coveralls]

Pull Request Test Coverage Report for Build 1951

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 95.463%

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Totals
Change from base Build 1889:	0.005%
Covered Lines:	4902
Relevant Lines:	5135

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💛 - Coveralls