Remove newlines from docstring signature (#4735)

* Remove newlines from docstring signature

* Jean/dev (#1)

Replace newlines in arg values with spaces

* style: pre-commit fixes

* Don't use std::find_if for C++ 11 compatibility

* Avoid implicit char to bool conversion

* Test default arguments for line breaks

* style: pre-commit fixes

* Separate Eigen tests

* style: pre-commit fixes

* Fix merge

* Try importing numpy

* Avoid unreferenced variable in catch block

* style: pre-commit fixes

* Update squash function

* Reduce try block

* Additional test cases

* style: pre-commit fixes

* Put statement inside braces

* Move string into function body

* Rename repr for better readability. Make constr explicit.

* Add multiline string default argument test case

* style: pre-commit fixes

* Add std namespace, do not modify string repr

* Test for all space chars, test str repr not modified

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