Modernize code base

[sdispater]

This PR modernizes the code by doing the following:

• Remove the PYTHON2.7/3.5 compatibility layers
• Use proper type annotations

[sdispater]

@finswimmer Let me know if you still see something that needs changing.

[sdispater]

I am gonna go ahead and merge this.

The import issue has already been fixed on Poetry's side (python-poetry/poetry#3696) and the fix will be available in the next 1.1.5 release.

[nickray]

This removal breaks https://github.com/python-poetry/poetry/blob/6eaa2984d9fb51a837c4ab11643f4dd9d7828d4d/poetry/console/commands/config.py#L14 (current master).

[nickray]

This removal of the VersionConstraint import breaks https://github.com/python-poetry/poetry/blob/6eaa2984d9fb51a837c4ab11643f4dd9d7828d4d/poetry/repositories/repository.py#L5 (current master).

[nickray]

@sdispater: Besides the type annotations and Python version housecleaning, multiple changes in this PR modify the public API of poetry-core. Technically, these are semver-breaking changes. Is this intended? Is there a corresponding PR to poetry dealing with the fallout already?

[sdispater]

@nickray Thanks for sharing you concerns.

poetry-core is a PEP-517 build backend and is not indented to be used as a library – except by Poetry itself – for now. The only public API that matters at the moment are the PEP-517 hooks provided by poetry.core.masonry.api.

There is currently a work in progress branch (https://github.com/python-poetry/poetry/compare/modern-core) on Poetry's side to update the imports and code to account for these changes in poetry-core.