Fixes for sympy update

[kratman]

Description

SymPy updated and seems to no longer recognize PyBaMM symbols as strings.

It gets to this block (N.B. warning text was edited for readability):

    if not isinstance(a, str):
        try:
            a = str(a)
        except Exception as exc:
            raise SympifyError(a, exc)
        sympy_deprecation_warning("The string fallback in sympify() is deprecated.")

Then fails due to an exception. Even if it did convert to a string, it would have caused a deprecation warning.

Failing tests looked like this:

a = pybamm.Symbol("a", domain="test")
self.assertEqual(pybamm.Arcsinh(a).to_equation(), sympy.asinh(a))

There seemed to be two options:

1. Add a pybamm.Symbol._sympy_() method. This would be basically identical to pybamm.Symbol.to_equation(), and would not be a good test.
2. Explicitly put the expected string into the sympy function. This works as a test of pybamm.Symbol.to_equation() by confirming that pybamm.Symbol.print_name() returns the original name, but could be a bit redundant.

I went with option (2) since it keeps the tests working, but it could be good to determine if pybamm.Symbol.print_name() and pybamm.Symbol.to_equation() could be just replaced by a pybamm.Symbol._sympy_()method.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

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

[kratman]

When this passes CI then it should fix the CI in #4250 and replace #4252

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.54%. Comparing base (4486522) to head (2cb22dd).
Report is 216 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4253      +/-   ##
===========================================
- Coverage    99.55%   99.54%   -0.02%     
===========================================
  Files          288      288              
  Lines        21886    21886              
===========================================
- Hits         21789    21786       -3     
- Misses          97      100       +3     

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[brosaplanella]

Looks good, happy to approve once coverage passes.

[kratman]

@brosaplanella I am pretty sure that failure is a false positive. It is the same failure in #4252, which does not change any code