Convert UTF-8 with BOM to UTF-8 with no BOM in Python

Question

Two questions here. I have a set of files which are usually UTF-8 with BOM. I'd like to convert them (ideally in place) to UTF-8 with no BOM. It seems like codecs.StreamRecoder(stream, encode, decode, Reader, Writer, errors) would handle this. But I don't really see any good examples on usage. Would this be the best way to handle this?

source files:
Tue Jan 17$ file brh-m-157.json 
brh-m-157.json: UTF-8 Unicode (with BOM) text

Also, it would be ideal if we could handle different input encoding wihtout explicitly knowing (seen ASCII and UTF-16). It seems like this should all be feasible. Is there a solution that can take any known Python encoding and output as UTF-8 without BOM?

edit 1 proposed sol'n from below (thanks!)

fp = open('brh-m-157.json','rw')
s = fp.read()
u = s.decode('utf-8-sig')
s = u.encode('utf-8')
print fp.encoding  
fp.write(s)

This gives me the following error:

IOError: [Errno 9] Bad file descriptor

Newsflash

I'm being told in comments that the mistake is I open the file with mode 'rw' instead of 'r+'/'r+b', so I should eventually re-edit my question and remove the solved part.