ignoring their transaction (txn) argument.

* trap NotImplementedError raised by os.urandom calls when not available
  on a particular system.

empty final matches with finditer(). New test cases included
for this bug and for #581080.

some escaping bugs in SRE.

this is the right way to document such things (Fred, help me out here :-),
but I got misled by the existing documentation and assumed the parameter
list was a *args sort of thing.

[Bug #1011606] Only check file descriptors for exceptional conditions if the fd is readable or writable

(Contributed by Dima Dorfman)

- wrap long line