Type annotations

[Pliner]

What do these changes do?

This PR adds type hints for aiopg except aiopg.sa. Due to cyclic dependencies between cursor, connection and transaction, I had to merge three files into one. The same thing is with context managers: due to cyclic dependencies(context managers were very specific) I had to generalise as much as possible _ContextManager and move specific ones closer to their usage.

Related issue number

#314

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

Merging #813 (3980d99) into master (5e65277) will decrease coverage by 0.09%.
The diff coverage is 96.83%.

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
- Coverage   93.34%   93.25%   -0.10%     
==========================================
  Files          13       11       -2     
  Lines        1548     1526      -22     
  Branches      179      179              
==========================================
- Hits         1445     1423      -22     
+ Misses         73       72       -1     
- Partials       30       31       +1     

Impacted Files	Coverage Δ
aiopg/pool.py	94.46% <85.22%> (-5.54%)	⬇️
aiopg/utils.py	86.36% <91.17%> (+0.74%)	⬆️
aiopg/connection.py	95.81% <99.74%> (+4.41%)	⬆️
aiopg/__init__.py	100.00% <100.00%> (ø)
aiopg/sa/connection.py	88.03% <100.00%> (+0.41%)	⬆️
aiopg/sa/engine.py	99.13% <100.00%> (+<0.01%)	⬆️
aiopg/sa/result.py	90.77% <100.00%> (ø)

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