Add cythonized version of sequences.c

[houseofsuns]

First try at cython. I tested various aproaches, but ended up with the one used in the c version. I've got the feeling, that there is not that much that can be done here. This was most of the time a tad slower than the c version, but sometimes even faster.

[codecov-io]

Codecov Report

Merging #37 into master will decrease coverage by -0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master     #37     +/-   ##
========================================
- Coverage   71.89%   71.8%   -0.1%     
========================================
  Files          44      44             
  Lines        5135    5135             
  Branches     1080    1080             
========================================
- Hits         3692    3687      -5     
- Misses       1265    1270      +5     
  Partials      178     178

Impacted Files	Coverage Δ
snakeoil/sequences.py	96.29% <ø> (-3.09%)	❌

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[radhermit]

Sorry, I merged this a while ago and forgot to close this.