Python 3 support: use six for encoding and basestring

[chapmanb]

Works from dev branch, fixing two remaining items preventing tests from passing on Python 3 using six:

• UTF encoding when working with GFF input files
• basestring comparisons

[codecov[bot]]

Codecov Report

Merging #51 into dev will decrease coverage by 0.02%. The diff coverage is 44.44%.

@@            Coverage Diff            @@
##              dev     #51      +/-   ##
=========================================
- Coverage   68.12%   68.1%   -0.03%     
=========================================
  Files          33      33              
  Lines        3065    3069       +4     
=========================================
+ Hits         2088    2090       +2     
- Misses        977     979       +2

Impacted Files	Coverage Δ
mirtop/exporter/vcf.py	5.99% <20%> (+0.38%)	:arrow_up:
mirtop/libs/do.py	61.01% <75%> (+0.67%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 699b22f...4a4c171. Read the comment docs.

[lpantano]

Thanks a lot!