use the os.ErrDeadlineExceeded for stream deadline errors on Go 1.15

[marten-seemann]

Fixes #2552.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7c204d4). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #2734   +/-   ##
=========================================
  Coverage          ?   86.51%           
=========================================
  Files             ?      128           
  Lines             ?     9956           
  Branches          ?        0           
=========================================
  Hits              ?     8613           
  Misses            ?     1010           
  Partials          ?      333           

Impacted Files	Coverage Δ
stream.go	79.07% <ø> (ø)
stream_deadline_error.go	100.00% <100.00%> (ø)
stream_deadline_error_go115.go	100.00% <100.00%> (ø)

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