Type stub fixes

[nilehmann]

• Type annotations for getall were wrong. The default value should not be of type _T.
• Also improve type annotations for getone by defining overloads.

[nilehmann]

PR similar to #283, but the type annotations differ. This PR allows for a more permissive type.

[asvetlov]

thank you. I'll port your changes into #283

[codecov]

Codecov Report

Merging #284 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #284   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files           8        8           
  Lines         710      710           
=======================================
  Hits          707      707           
  Misses          3        3

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[asvetlov]

Done as part of #283