Fix indentation magic when `maximum_signature_line_length` is used

[pradyunsg]

Discussed in #749

x-ref #696 which is related, and sphinx-doc/sphinx#11011 which added this feature to Sphinx.

[Liam-DeVoe]

We just ran into this in Hypothesis (HypothesisWorks/hypothesis#4310), and used the css suggested by @tqbl in #749 to fix it (thanks!):

.sig:not(.sig-inline) {
    padding-left: 0.5em;
    text-indent: 0;
}