chore: make _types always present in RESTManager

[JohnVillalovos]

We now create _types = {} in RESTManager class.

By making _types always present in RESTManager it makes the code
simpler. We no longer have to do:
types = getattr(self, "_types", {})

And the type checker now understands the type.

[codecov-io]

Codecov Report

Merging #1365 (1b5cdc5) into master (48fc907) will increase coverage by 0.01%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master    #1365      +/-   ##
==========================================
+ Coverage   79.92%   79.94%   +0.01%     
==========================================
  Files          73       73              
  Lines        4010     4008       -2     
==========================================
- Hits         3205     3204       -1     
+ Misses        805      804       -1     

Flag	Coverage Δ
unit	79.94% <58.33%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gitlab/v4/cli.py	40.40% <0.00%> (+0.16%)	⬆️
gitlab/mixins.py	77.77% <62.50%> (-0.16%)	⬇️
gitlab/base.py	84.86% <100.00%> (+0.20%)	⬆️

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[nejch]

Cool, gets rid of some of the magic as well.