Update python syntax in OAuth2 example

[headcr4sh]

Since Python 2.x has been retired, it makes sense to update code examples to the new Python 3 syntax.
Sidenote: Invoking print as a function (instead of using the depreacted Python 2.x built-in) is also backwards compatible to Python 2, so this is by no means a "breaking change".

What this PR does / why we need it:

Improvement of the documentation

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• None of the above. ;-)

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[headcr4sh]

Python 3 changed the way in which bytes are being printed.
Therefore the addional decode invocation is needed to avoid strings that look like b'__base64_encoded_data__'.

[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@headcr4sh thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, headcr4sh

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[codecov-io]

Codecov Report

Merging #5000 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #5000      +/-   ##
=========================================
- Coverage   58.52%   58.5%   -0.03%     
=========================================
  Files          88      88              
  Lines        6783    6791       +8     
=========================================
+ Hits         3970    3973       +3     
- Misses       2377    2380       +3     
- Partials      436     438       +2

Impacted Files	Coverage Δ
internal/ingress/controller/controller.go	49.93% <0%> (-0.25%)	⬇️
internal/watch/file_watcher.go	84.61% <0%> (+3.84%)	⬆️

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[aledbf]

/retest