Change method for getting default user name, some sudo versions unexcepted working with getpass

[AndrewBille]

For example sudo version 1.8.25p1: [root@centos-8 ~]# sudo -u test sh -c 'python3 -c "import getpass; print(getpass.getuser())"'; root [root@centos-8 ~]# sudo -u test sh -c 'python3 -c "import os; print(os.getlogin())"'; test

[codecov[bot]]

Codecov Report

Merging #67 (468b877) into master (4024049) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   95.93%   95.93%           
=======================================
  Files          18       18           
  Lines        1674     1674           
=======================================
  Hits         1606     1606           
  Misses         68       68           

Impacted Files	Coverage Δ
testgres/defaults.py	100.00% <100.00%> (ø)

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