Python version dependency in pyproject.toml breaks poetry for modified Python versions

[gcbirzan]

Describe the bug
Trying to create a new interpreter on Debian with python3.9 generates invalid pyproject.toml

To Reproduce
Steps to reproduce the behavior:

1. Install python3.9 on Debian.
2. Try to generate a virtualenv based on that
3. Error:

ParseConstraintError

  Could not parse version constraint: ^3.9.1+

  at ~/.poetry/lib/poetry/_vendor/py3.6/poetry/core/semver/__init__.py:164 in parse_single_constraint
      160│         else:
      161│             return version
      162│ 
      163│     raise ParseConstraintError(
    → 164│         "Could not parse version constraint: {}".format(constraint)
      165│     )
      166│ 

This is because the version of the Debian Python is 3.9.1+ and that gets put in the deps:

[tool.poetry.dependencies]
python = "^3.9.1+"

Expected behavior
poetry init suggests ^3.9, that's probably a more sensible value.

Environments (please complete the following information):
Python 3.9 is only available on testing/sid, but it should still work.

[koxudaxi]

@gcbirzan
Thank you for posting this problem.
I understand the problem.
I don't expect the version type.
I will fix it.

[koxudaxi]

@gcbirzan
The latest version 1.0.7 has been published in the market.
The version only picks a major and minor version like 3.6.