Fix unstable SAML test

[KOliver94]

Proposed changes

After investing a lot of time into this issue, I realised it appears to happen only on Linux and not on Windows. I started to debug the methods and calls and it seemed to be an issue with XML parsing. The latest successful CI run used lxml version 4.6.4 and after version 4.7.1 it started to break. I did not inverstigate what could cause this seemingly platform dependent issue but there is also an open issue in python3-saml. I chose the easier path and pinned the version of lxml under 4.7.0 which seems to solve the issue.

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Codecov Report

Merging #653 (5378daa) into master (1ea27e8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #653   +/-   ##
=======================================
  Coverage   76.92%   76.92%           
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  Files         317      317           
  Lines        9630     9630           
  Branches      962      962           
=======================================
  Hits         7408     7408           
  Misses       2071     2071           
  Partials      151      151           

Flag	Coverage Δ
unittests	76.92% <ø> (ø)

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[nijel]

Merged, thanks for your contribution!