Micro optimization: move Python version check from function exection time to import time

[asvetlov]

No description provided.

[asvetlov]

The change is very subtle, I don't expect the real speedup for real applications (not benchmarks).
On the other hand, the PR is clear and obvious, it doesn't make the code harder.

[codecov]

Codecov Report

Merging #236 (8266352) into master (9ea9f9b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #236   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          107       109    +2     
  Branches        16        16           
=========================================
+ Hits           107       109    +2     

Flag	Coverage Δ
unit	98.16% <66.66%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
async_timeout/__init__.py	100.00% <100.00%> (ø)

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