Fix xml indent() on Python 3.6

[dmach]

No description provided.

[codecov]

Codecov Report

Patch coverage: 22.58% and project coverage change: -0.13 ⚠️

Comparison is base (3559336) 28.98% compared to head (658f2ac) 28.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1361      +/-   ##
==========================================
- Coverage   28.98%   28.86%   -0.13%     
==========================================
  Files          36       45       +9     
  Lines       16729    17062     +333     
==========================================
+ Hits         4849     4925      +76     
- Misses      11880    12137     +257     

Impacted Files	Coverage Δ
osc/_private/project.py	0.00% <0.00%> (ø)
osc/commandline.py	19.01% <0.00%> (+<0.01%)	⬆️
osc/commands/repo.py	0.00% <0.00%> (ø)
osc/commands/repo_add.py	0.00% <0.00%> (ø)
osc/commands/repo_list.py	0.00% <0.00%> (ø)
osc/commands/repo_remove.py	0.00% <0.00%> (ø)
osc/_private/api.py	46.66% <8.00%> (-14.88%)	⬇️
osc/core.py	35.59% <14.28%> (-0.06%)	⬇️
osc/output/tty.py	46.66% <46.66%> (ø)
osc/output/key_value_table.py	86.79% <86.79%> (ø)
... and 3 more

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