Do not allow async import for Python < 3.8

[tfarago]

We can either do this or bump the minimum python version. If the former:

• disable non-interactive top level await for Python < 3.8

[codecov]

Codecov Report

Merging #500 (f3b28a5) into master (07762b4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #500   +/-   ##
=======================================
  Coverage   87.75%   87.75%           
=======================================
  Files         115      115           
  Lines        7931     7931           
=======================================
  Hits         6960     6960           
  Misses        971      971           

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[MarcusZuber]

I don't have a running python3.7, but it looks fine.