poetry shell does not send a newline to fish

[NiklasKappel]

• Poetry version: Poetry (version 1.4.2)
• Python version: 3.11.3
• OS version and name: Fedora 38 Silverblue
• pyproject.toml: https://github.com/python-poetry/poetry/blob/master/pyproject.toml

• I am on the latest stable Poetry version, installed using a recommended method.
• I have searched the issues of this repo and believe that this is not a duplicate.
• I have consulted the FAQ and blog for any relevant entries or release notes.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option) and have included the output below.

Issue

Steps to reproduce:

fish
cd poetry/
poetry shell

Observed behavior ("|" represents cursor position):

[niklas@toolbox poetry]$ poetry shell
Spawning shell within /var/home/niklas/.cache/pypoetry/virtualenvs/poetry-Xjwftcq8-py3.11
[niklas@toolbox poetry]$ source /var/home/niklas/.cache/pypoetry/virtualenvs/poetry-Xjwftcq8-py3.11/bin/activate.fish|

Expected behavior:

[niklas@toolbox poetry]$ poetry shell
Spawning shell within /var/home/niklas/.cache/pypoetry/virtualenvs/poetry-Xjwftcq8-py3.11
[niklas@toolbox poetry]$ source /var/home/niklas/.cache/pypoetry/virtualenvs/poetry-Xjwftcq8-py3.11/bin/activate.fish
(poetry-py3.11) [niklas@toolbox poetry]$ |

In fish, the source command is not executed automatically when running poetry shell. The reason is that fish does not count os.linesep as a return/enter when the sendline method of pexpect is used. Compare here.

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