Temporarily pin tox < 4

[Saransh-cpp]

Description

#2537 might take some time, given that there are a lot of breaking changes and not all of them are documented well. Additionally, they have enabled isolated_builds to be always true, which I think is the reason for the failing ubuntu tests -

The macOS and windows tests work with v4 now, but the ubuntu tests don't.

This PR aims to temporarily fix the CI by upper-pinning tox (till the time tox.ini is migrated to v4).

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[valentinsulzer]

Thanks, looks like this worked!

[codecov]

Codecov Report

Base: 99.72% // Head: 99.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3961286) compared to base (b072d22).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #2541    +/-   ##
=========================================
  Coverage    99.72%   99.72%            
=========================================
  Files          268      270     +2     
  Lines        19315    19423   +108     
=========================================
+ Hits         19262    19370   +108     
  Misses          53       53            

Impacted Files	Coverage Δ
pybamm/parameters/__init__.py	100.00% <0.00%> (ø)
pybamm/__init__.py	100.00% <0.00%> (ø)

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