Version constraint prevents installation via Poetry

[wanderrful]

Describe the bug

Using Python ^3.7 or ^3.8 in your pyproject.toml file (i.e. the default values that Poetry generates) prevents adding this package via poetry add:

[SolverProblemError]
The current project's Python requirement (^3.8) is not compatible with some of the required packages Python requirement:
  - flake8-annotations requires Python >=3.6.1,<3.9

Because no versions of flake8-annotations match >2.2.0,<3.0.0
 and flake8-annotations (2.2.0) requires Python >=3.6.1,<3.9, flake8-annotations is forbidden.
So, because tool-belt depends on flake8-annotations (^2.2.0), version solving failed.

To Reproduce

• poetry init --no-interaction to initialize a new Poetry project.
• The python version in your pyproject.toml file will be ^3.8 or ^3.7 by default
• poetry add flake8 (works fine)
• poetry add flake8-annotations (throws the error described above)

Version Information

$ flake8 --version
3.8.3 (black: 0.2.0, flake8-bandit: 2.1.2, flake8-bugbear: 20.1.4, import-
order: 0.18.1, mccabe: 0.6.1, pycodestyle: 2.6.0, pyflakes: 2.2.0) CPython
3.8.2 on Darwin

$ python -V
Python 3.8.2

[sco1]

Thanks for the ping

Looks like this has also been asked upstream: python-poetry/poetry#2646
And answered: python-poetry/poetry#1930 (comment)

The default caret requirement of, say ^3.8, tells Poetry that your project is compatible with [3.8, 4.0), but we currently do not guarantee compatibility with >=3.9 so this is technically an incompatibility. This can be solved with a tilde requirement, ~3.8, which would describe compatibility with [3.8, 3.9).

While our dependency pinning change (see: e2f9452) is technically correct, I'm not sure it's worthwhile to insist on this if it clashes with existing project specs; in principle it's worthwhile, but it's not a hill worth dying on.

I'll try and revert the change this afternoon.