Wrong metadata generated for old Python versions

[tueda]

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Ubuntu 20.04.2 LTS / WSL2
• Poetry version: 1.1.5
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/tueda/4c591340c44139b31093a84c4e63d05e#file-pyproject-toml

Issue

Although Poetry doesn't run with old Python versions like 2.6, as far as I understand, developing packages supporting Python 2.6 should be still supported by Poetry (running with a newer Python).

But it seems that source distributions and wheels generated by poetry build have wrong metadata for old Python versions.
Example:

[tool.poetry.dependencies]
python = "~2.6 || ~2.7 || ^3.2"

Poetry generates:

Metadata-Version: 2.1
Name: example
Version: 0.1.0
Summary: 
Author: Your Name
Author-email: you@example.com
Requires-Python: >=2.7, !=3.0.*, !=3.1.*
Classifier: Programming Language :: Python :: 2
Classifier: Programming Language :: Python :: 2.7
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.4
Classifier: Programming Language :: Python :: 3.5
Classifier: Programming Language :: Python :: 3.6
Classifier: Programming Language :: Python :: 3.7
Classifier: Programming Language :: Python :: 3.8
Classifier: Programming Language :: Python :: 3.9

So, 2.6 is omitted in Requires-Python (same for python_requires in setup.py). 3.2 and 3.3 are also missing in the classifiers.

[finswimmer]

The problem is that we hardcode the available python versions here: https://github.com/python-poetry/poetry-core/blob/8d5e5c5d070940736ab72c5dec09cd7deab07cf3/poetry/core/packages/package.py#L31

We have another place where we do this: https://github.com/python-poetry/poetry-core/blob/8d5e5c5d070940736ab72c5dec09cd7deab07cf3/poetry/core/version/helpers.py#L12

I'm not sure if there is a better way to implement it 🤔

[jleclanche]

Im not sure this is the same issue, but we're seeing similar issues in dj-stripe.

dj-stripe/dj-stripe#1647

the python 3.7 classifier is not generated with python = "^3.7.12".

[dimbleby]

@jleclanche it's easy to fix poetry to behave the way you expect, if you can find some sort of reference to confirm that the intended meaning of (say) the "Python :: 3.7" classifier is to cover any python 3.7.x then I expect a merge request would be accepted

--- a/src/poetry/core/packages/package.py
+++ b/src/poetry/core/packages/package.py
@@ -297,10 +297,7 @@ class Package(PackageSpecification):
         for version in sorted(
             self.AVAILABLE_PYTHONS, key=lambda x: tuple(map(int, x.split(".")))
         ):
-            if len(version) == 1:
-                constraint = parse_constraint(version + ".*")
-            else:
-                constraint = Version.parse(version)
+            constraint = parse_constraint(version + ".*")