CI: code check for " " introduced by black

[jbrockmendel]

Sometimes when we run black on something like

foo = (
    "bar "
    "baz"
)       

we end up with foo = "bar " "baz" when we would want foo = "bar baz".

This is not considered a bug in black, so is something we're currently checking for manually. We should add it to code_checks.

[ShaharNaveh]

take