Migrant

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- Nothing changed yet.

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- Ability to skip database from migration, when it not available.

- API change: to support parallel processing Backend class grew connection
management methods, that needs to be implemented in plugins.

- Type annotations added

- Support migration of multiple databases in parallel (see `--parallel`
command line argument for command)

- Add support for Python-3.8

- Drop support for Python-2.7

- Modernize development environment with pipenv

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- Updated the script template to make it lint friendly.

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- Suppport for Python 3.6 and 3.7. Dropped Python 3.5.

- Fixed compatibility to not use `future`, since it imported `ConfigParser` as
`configparser` in Python 2.7. Instead the official `configparser` backport
is used.

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- Bugfix: proper error when `cmd` argument missing in Python 3

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- Python 3 compatibility.