Fix zip usage for compatibility with python 3

[mxmpl]

In Python 3, zip returns an iterator and not a list as in Python 2: it exhausts once it has been iterated over.

This behaviour causes issues in

abkhazia/abkhazia/corpus/corpus_merge_wavs.py

Lines 87 to 89 in 79632be

	for spkr in self.speakers:
	spk_utts = [utt_id for utt_id, utt_speaker in self.utts
	if utt_speaker == spkr]

as self.utts is defined in

abkhazia/abkhazia/corpus/corpus_merge_wavs.py

Line 61 in 79632be

self.utts = zip(utt_ids, utt_speakers)

For the first spkr, spk_utts is correct, but for the next ones it is an empty list as self.utts is exhausted.

This PR fixes this by changing zip(...) to list(zip(...)).