python-pexpect: fix byte-compilation with Python 2

python-pexpect: fix byte-compilation with Python 2

pexpect contains one file, async.py, that is only meant to be used
with Python 3. Due to this, it fails to byte-compile with Python 2,
which will become a problem once we make byte-compilation failures
fatal failures that abort the build.

As suggested by upstream at
https://github.com/pexpect/pexpect/issues/290, we simply remove the
problematic file when building with Python 2.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>