join together duplicate entries in the text repr

[keewis]

Indexes contains one entry per coordinate, even if there are indexes that index multiple coordinates. This deduplicates the entries, but the format is not quite clear.

• follow-up to display the indexes in the string reprs #6795
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

The formatting options we were able to come up with:

1. separate with just newlines (see e29aeb9):

Indexes:
    one    CustomIndex
    two
    three  PandasIndex

2. mark unique indexes with a prefix to make it look like a list (see 9b90f8b):

Indexes:
  - one    CustomIndex
    two
  - three  PandasIndex

3. use unicode box components (in front of the coordinate names) (see 2cec070):

Indexes:
  ┌ one    CustomIndex
  │ two
  └ three
    four  PandasIndex

4. use unicode box components (after the coordinate names) (see 492ab47):

Indexes:
    one   ┐ CustomIndex
    two   │
    three ┘
    four    PandasIndex

For the unicode box components, we can choose between the light and heavy variants.

@benbovy and I think the unicode variants (especially variant 3) are the easiest to understand, but we would need to decide whether we care about terminals that don't support unicode.

Edit: in the meeting we decided that support for the subsection of unicode should be common enough that we can use it. I'll clean this PR up to implement option 3, then.

[headtr1ck]

Some tests and this looks good.

[keewis]

@headtr1ck, any ideas how to resolve the mypy errors related to the monkeypatching in the test?

[keewis]

the typing (especially the typing ignores of the monkeypatching in the test) could use another review, but other than that this should be ready for merging.

[keewis]

there appears to be something wrong with the windows CI (all python versions): the "version info" step, which usually takes about 10 seconds, has been running for more than 4 hours now (and this is not just this PR, #7999 is affected, as well)

[dcherian]

Thanks @keewis