Poetry: Poetry 1.0.10 seems to use the wrong version of Python when running `poetry install` in mixed Python2/3 environment

[taranlu-houzz]

• [x] I am on the latest Poetry version.
• [x] I have searched the issues of this repo and believe that this is not a duplicate.

• [x] If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: macOS 10.14.6

• Poetry version: 1.0.10

Issue

I have the following pyenv setup:

❯ pyenv versions
  system
* 2.7.17 (set by /Users/taran_lu/.pyenv/version)
* 3.7.7 (set by /Users/taran_lu/.pyenv/version)
  3.8.3

Globally, python is set to Python 2.7.17 installed via pyenv and python3 is set to Python 3.7.7 also via pyenv. In the previous version of poetry, when I would run poetry install in a project that was Python 3 only, poetry would detect that current version of Python did not match the requirements, after which, it would search and find the pyenv version of Python 3 and use that successfully.

After updating to 1.0.10, poetry install now fails with the following error:

❯ poetry install
The currently activated Python version 2.7.17 is not supported by the project (^3.7,<3.9).
Trying to find and use a compatible version.
Using python3 (3.7.7)
Creating virtualenv --- in /---/.venv
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'PosixPath' is not defined
Installing dependencies from lock file

[SolverProblemError]
The current project's Python requirement (2.7.17) is not compatible with some of the required packages Python requirement:
  - pygments requires Python >=3.5

Because no versions of bpython match >0.19,<0.20
 and bpython (0.19) depends on pygments (*), bpython (>=0.19,<0.20) requires pygments (*).
Because pygments (2.6.1) requires Python >=3.5
 and no versions of pygments match !=2.6.1, pygments is forbidden.
Thus, bpython is forbidden.
So, because --- depends on bpython (^0.19), version solving failed.

Somehow, it seems to be getting confused about the project's requirements.

[smalllark]

Workaround: install poetry < 1.0.10

[taranlu-houzz]

@smalllark Yeah, that is what I have ended up doing for now.

[mariofix]

Also, latest beta fixes it

```
$ poetry -V
Poetry version 1.0.10
$ poetry self update --preview
/my_personal_dir/.poetry/lib/poetry/_vendor/py2.7/subprocess32.py:149: RuntimeWarning: The _posixsubprocess module is not being used. Child process reliability may suffer if your program uses threads.
"program uses threads.", RuntimeWarning)
Updating to 1.1.0b2

• Downloading poetry-1.1.0b2-linux.tar.gz 100%

Poetry (1.1.0b2) is installed now. Great!
$ poetry -V
Poetry version 1.1.0b2
$ poetry env use 3.7
Creating virtualenv lala-9oGnfBL0-py3.7 in /my_personal_dir/.cache/pypoetry/virtualenvs
Using virtualenv: /my_personal_dir/.cache/pypoetry/virtualenvs/lala-9oGnfBL0-py3.7
$ poetry install
Installing dependencies from lock file

Package operations: 42 installs, 0 updates, 0 removals

``

[Seanny123]

@taranlu-houzz possibly resolved with the release of 1.1.0?