Improve handling of strings supplied as matrix entries

[DaveWitteMorris]

Fixes #34821. The construction matrix(ZZ, nrows=2, entries=["12", "12"]) is ambiguous, because the string "12" could be interpreted either as the single integer 12 or as a sequence of two single-digit integers: that is, entries could be interpreted as either [12, 12] or [[1,2], [1,2]]. To avoid this problem, the sequence_type method of MatrixArgs (in sage.matrix.args.pyx) does not allow strings as the entries of certain matrices with only one column (and in some other situations):

# Blacklist strings, we don't want them to be considered a sequence
        return MA_ENTRIES_UNKNOWN

However, this special treatment of one-column matrices results in inconsistent (and very puzzling) behaviour, such as the following:

# current behaviour
sage: matrix(ZZ, 1,2, ["1", "2"])
[1 2]
sage: matrix(ZZ, 2,1, ["1", "2"])
TypeError: unable to convert ['1', '2'] to a matrix

This PR eliminates the inconsistency, by interpreting the string as a scalar (i.e., an element of the ring) in this situation, which is the same interpretation as when there is more than one column.

# after this PR
sage: matrix(ZZ, 2,1, ["1", "2"])
[1]
[2]

The PR also makes a similar change in the get_type method, in order to eliminate the following TypeError:

# current behaviour
sage: matrix(ZZ, 2, 2, 3)
[3 0]
[0 3]
sage: matrix(ZZ, 2, 2, "3")
TypeError: unable to convert '3' to a matrix

Associated doctests are added/updated.

📝 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.
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[orlitzky]

It's crazy to accept strings at all, but better to be consistently crazy in that case Thanks!