LaTeX: legacy \pysiglinewithargsret does not compute correctly available horizontal space and should use a ragged right style

[jfbu]

Describe the bug
A clear and concise description of what the bug is.

Here is a screenshot from PDF rendering of Sphinx docs:

This is taken from the section documenting the APIs to the autodoc extension.

We observe:

• some stretch of whitespace in signature arguments,
• hyphenation of Tuple but there seems to be space available

This is explained from the latex code which has a wrong computation of avaiable horizontal space; and the parbox should use \raggedright. This is will reduce hyphenation of keywords also.

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]
• Python version: [e.g. 3.7.1]
• Sphinx version: applies to all versions at least since 1.5 say
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: TeXLive 2021

Additional context
Add any other context about the problem here.

• Relates indirectly to No wrapping for cpp:enumerator #7241 and missing line break in pysigline #8980. To fix the later one should use \parbox as indicated in No wrapping for cpp:enumerator #7241 (comment), and this has the correct computation of available width which is wrong in \pysiglinewithargsret currently.

With some fixes the above screenshot would look like this

It does not go all the way to right end (which would match docu-) as it is know that \raggedright makes hyphenation very unlikely, but this is a good thing, not to hyphenate too easily in signature, I think.

[jfbu]

Fixed at 2934aac