[Python] Update checkDsymForUUIDIsOn to be compatible with Python 3.

Summary:
In python 2, strings and bytes are the same, but they're not in
python 3, hence the return of read() needs an explicit conversion.
While I'm around, rename the return of Popen() from `pipe` to
`process`, as that's what Popen returns.

Reviewers: JDevlieghere, friss, zturner, aprantl, serge-sans-paille

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D56517

llvm-svn: 350788