[DPE-6042] Make tox commands resilient to white-space paths

[sinclert-canonical]

This PR fixes tox.ini commands when the repository is cloned on a white-space containing path.

How to reproduce:

$ mkdir -p "Projects/Canonical/Data Platform/PostgreSQL"
$ cd "Projects/Canonical/Data Platform/PostgreSQL"
$ git clone https://github.com/canonical/postgresql-k8s-operator
$ cd postgresql-k8s-operator
$ tox run -e format
> ...
> /Users/<USERNAME>/Projects/Canonical/Data:1:1: E902 No such file or directory (os error 2)
> Platform/PostgreSQL/postgresql-k8s-operator/src:1:1: E902 No such file or directory (os error 2)
> Platform/PostgreSQL/postgresql-k8s-operator/tests:1:1: E902 No such file or directory (os error 2)

Additional considerations

Using the quoted paths to set up the PYTHONPATH does not work. This can be tested by running the unit tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.46%. Comparing base (5852221) to head (eb6d40c).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #780      +/-   ##
==========================================
- Coverage   75.60%   75.46%   -0.15%     
==========================================
  Files          12       12              
  Lines        3116     3118       +2     
  Branches      475      476       +1     
==========================================
- Hits         2356     2353       -3     
- Misses        617      622       +5     
  Partials      143      143              

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