requirements: bump attrs 21.3.0 -> 21.4.0

[Bastian-Krause]

Description
attrs 21.3.0 is breaking code coverage [1].

To fix this, bump attrs to 21.4.0.

[1] python-attrs/attrs#895

Checklist

• PR has been tested (test-suite only)

See #1029

[codecov]

Codecov Report

Base: 0.0% // Head: 62.6% // Increases project coverage by +62.6% 🎉

Coverage data is based on head (deb5594) compared to base (ca45665).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1032      +/-   ##
=========================================
+ Coverage        0   62.6%   +62.6%     
=========================================
  Files           0     151     +151     
  Lines           0   11383   +11383     
=========================================
+ Hits            0    7127    +7127     
- Misses          0    4256    +4256     

Impacted Files	Coverage Δ
labgrid/util/exceptions.py	0.0% <0.0%> (ø)
labgrid/driver/networkinterfacedriver.py	70.3% <0.0%> (ø)
labgrid/driver/qemudriver.py	74.5% <0.0%> (ø)
labgrid/protocol/commandprotocol.py	100.0% <0.0%> (ø)
labgrid/util/timeout.py	100.0% <0.0%> (ø)
labgrid/protocol/videoprotocol.py	100.0% <0.0%> (ø)
labgrid/util/dict.py	100.0% <0.0%> (ø)
labgrid/resource/udev.py	50.8% <0.0%> (ø)
labgrid/protocol/resetprotocol.py	100.0% <0.0%> (ø)
labgrid/driver/httpvideodriver.py	56.0% <0.0%> (ø)
... and 141 more

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