chore: fix tox configuration

[hadrien]

Description

• Former CI config was actually running tests against the sqlalchemy version installed by poetry;
• Changed it so that dependencies are installed by tox using pyproject.toml: this is the standard way IIUC;
• Today I learnt by reading on [FAQ]: The tox answer should recommend poetry install --no-root instead of poetry install python-poetry/poetry#1941 that [dev.dependencies] section is not standard/compliant with pep-517;
• It is a chore as it doesn't fix anything in the library

Related PRs

• blocking feat: port to SQLAlchemy 1.4 / 2.0 #28

[codecov]

Codecov Report

Merging #27 (c2e5e54) into master (fc8a6ef) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #27   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          128       128           
  Branches         1         1           
=========================================
  Hits           128       128           

Flag	Coverage Δ
sqlalchemy1.3	100.00% <ø> (ø)

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[vicrep]

TIL about that PEP. Nice one 👍