Add type annotations

[asvetlov]

Fixes #265

[codecov]

Codecov Report

Merging #285 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #285   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         105    114    +9     
  Branches       17     17           
=====================================
+ Hits          105    114    +9

Impacted Files	Coverage Δ
aiohttp_jinja2/__init__.py	100% <100%> (ø)	⬆️
aiohttp_jinja2/helpers.py	100% <100%> (ø)	⬆️

---

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 386ea1e...d482c82. Read the comment docs.