use net.ErrClosed (for Go 1.16)

[marten-seemann]

Fixes #2933.

@mvdan, can I ask for your review here?

[codecov]

Codecov Report

Merging #3163 (b384982) into master (b8f07c7) will decrease coverage by 0.01%.
The diff coverage is 88.89%.

@@            Coverage Diff             @@
##           master    #3163      +/-   ##
==========================================
- Coverage   85.43%   85.43%   -0.01%     
==========================================
  Files         132      134       +2     
  Lines        9824     9848      +24     
==========================================
+ Hits         8393     8413      +20     
- Misses       1058     1060       +2     
- Partials      373      375       +2     

Impacted Files	Coverage Δ
internal/qerr/errors.go	100.00% <ø> (ø)
internal/qerr/errors_go116.go	83.33% <83.33%> (ø)
internal/qerr/errors_not_go116.go	100.00% <100.00%> (ø)

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[marten-seemann]

Merging, as we need to cut a release.