fix testenv once coverage in docs

In the file,the basepython value has been
set for the specified virtual environment.
So we don't need to set up an override in
testenv.
For this change, we executed the tox
command for testing to ensure that the
modification did not produce errors.

Change-Id: I290867f0b062e5b3b6327fb44dbf759bd857c86d
Closes-Bug: #1835394

tox.ini

@@ -4,7 +4,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1