Remove trailing whitespace on first line of multiline string.

[lrhn]

Proof-of-concept removal of non-participating trailing space and backslashes from the first line of multiline strings.

Slightly dangerous, becuase it had to tweak the "changes only whitespace" checking to allow removing backslashes (only precedeing a whitespace). That could allow removing the backslash in a raw string literal like r"\ " without being noticed.
(I know a current experiment also allows removing commas, so maybe it's easier to integrate into that experiment, if it has already generalized the "check whitespace only changes" test.)

[munificent]

Thanks for doing this! I like cleaning up the leading whitespace, but not normalizing escapes. Assuming you're OK with removing the code for cleaning up the escapes, LGTM after that.

[munificent]

Oh, one more thing. Before merging this, please add an entry describing the change to the CHANGELOG. Thanks!

[lrhn]

PTAL (did some significant changes to behavior, so another look is probably warranted.)

[lrhn]

Merged to head, updated changelog to be at next version. Tests still succeed, so landing.