Add i8 as alias for 'byte'

[zhaakhi]

THRIFT-3393 added i8 as an alias for byte, for consistency with the other integer types.

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   80.35%   80.46%   +0.11%     
==========================================
  Files          35       35              
  Lines        3476     3476              
==========================================
+ Hits         2793     2797       +4     
+ Misses        683      679       -4

Impacted Files	Coverage Δ
thriftpy2/parser/lexer.py	87.5% <ø> (ø)	⬆️
thriftpy2/parser/parser.py	95.44% <100%> (+0.82%)	⬆️

---

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 a6caab3...18c275f. Read the comment docs.

[ethe]

LGTM