Fix venv wrapper reset retval error with gym env

[Trinkle23897]

Close #697

[codecov-commenter]

Codecov Report

Merging #712 (d384cc2) into master (f270e88) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #712   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files          72       72           
  Lines        4895     4895           
=======================================
  Hits         4564     4564           
  Misses        331      331           

Flag	Coverage Δ
unittests	93.23% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tianshou/data/collector.py	94.24% <ø> (ø)
tianshou/env/venvs.py	94.07% <ø> (ø)
tianshou/env/venv_wrappers.py	83.33% <100.00%> (ø)

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more