I've found that having a way to strip a leading substring from a string
is convienent. I've also gotten bitten by the fact that str.strip takes
a sequence of characters to remove from the beginning -- not a full string.

I've attached a patch that implements lstrips, rstrips, and strips on
str, unicode, and as methods in the string module. I'm not particularly
attached to the names.

Please consider this patch for inclusion in Python. Thanks!