MNT/DOC: Make it clear this is py3 only

[ericdill]

closes #42

[parente]

🎉 for python3!

[codecov]

Codecov Report

Merging #44 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   94.09%   94.06%   -0.03%     
==========================================
  Files           2        2              
  Lines         220      219       -1     
==========================================
- Hits          207      206       -1     
  Misses         13       13

Impacted Files	Coverage Δ
conda_mirror/conda_mirror.py	93.98% <ø> (-0.03%)	⬇️

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[jhoblitt]

👍