Make Sequence pretty-printed in IPython

[user202729]

Related to #36801 , but this does not resolve that one. Instead:

• modify the function to respect _repr_pretty_ just like IPython
• implement _repr_pretty_ for Sequence and PolynomialSequence
• Add notes that _repr_ and cr= and cr_str= of Sequence are mostly unused
• Add # need sage.groups (automatic fix by sage-fixdoctests)

When #36801 is implemented part of this would be redundant (because Sequence derives from list)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.
• fix doc tests
• explain meaning of cr=True in Sequence constructor

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 2d5ce6d; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[tobiasdiez]

In general, I like the new output more. Especially:

File "src/sage/tests/books/computational-mathematics-with-sagemath/linalg_doctest.py", line 402, in sage.tests.books.computational-mathematics-with-sagemath.linalg_doctest
Failed example:
    A.eigenvectors_right()
Expected:
    [(4, [
    (1, 5, 5, 1)
    ], 1), (1, [
    (0, 1, 1, 4)
    ], 1), (2, [
    (1, 3, 0, 1),
    (0, 0, 1, 1)
    ], 2)]
Got:
    [(4, [(1, 5, 5, 1)], 1),
     (1, [(0, 1, 1, 4)], 1),
     (2, [(1, 3, 0, 1), (0, 0, 1, 1)], 2)]

Since its also aligns well with standard IPython behavior, I would say go ahead and fix the tests so we can merge it.

[user202729]

#39058 bites again.

---

The code actually become quite ugly once I notice PolynomialSequence exists. Will leave a note there for the thing to be cleaned up eventually when #36801 is fixed.

On another note, should we deprecate cr= and cr_str=?

[user202729]

I realize this is rather problematic: the current behavior ignores _repr_pretty_ (which IPython supports), and the representation logic of PolynomialSequence is rather complicated:

        if len(self) < 20:
            return Sequence_generic._repr_(self)
        else:
            return "Polynomial Sequence with %d Polynomials in %d Variables" % (len(self),self.nvariables())

I propose we add back _repr_pretty_ support, then move the current Sequence pretty printer (as well as PolynomialSequence) to the class.

Also avoid confusion like #26792 (comment)

[tobiasdiez]

LGTM

[user202729]

I just went through the whole diff and notice a handful of ... being changed automatically or # random being automatically removed by --fixdoctests… so I just add them back.