[issue32888] Improve exception message in ast.literal_eval

[Chris Angelico]

New submission from Chris Angelico <rosuav at gmail.com>:

When a non-literal is given to literal_eval, attempt to be more
helpful with the message, rather than calling it 'malformed'.

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components: Library (Lib)
messages: 312423
nosy: Rosuav
priority: normal
pull_requests: 5555
severity: normal
status: open
title: Improve exception message in ast.literal_eval
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32888>
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