remove dependency on package mock

[fabianegli]

The mock package is now part of the standard library of Python integrated into unittest. The mock package itself is not (regularly) updated any more. See https://pypi.org/project/mock/ (the latest release as of writing has been in 2020).

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1696 (bbcceeb) into dev (cc1ab61) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1696      +/-   ##
==========================================
- Coverage   67.99%   67.98%   -0.02%     
==========================================
  Files          56       56              
  Lines        6609     6694      +85     
==========================================
+ Hits         4494     4551      +57     
- Misses       2115     2143      +28     

Impacted Files	Coverage Δ
nf_core/lint/multiqc_config.py	66.66% <0.00%> (-14.59%)	⬇️
nf_core/lint/actions_awsfulltest.py	83.33% <0.00%> (-5.56%)	⬇️
nf_core/lint/readme.py	75.75% <0.00%> (-4.89%)	⬇️
nf_core/lint/actions_awstest.py	78.94% <0.00%> (-4.39%)	⬇️
nf_core/lint/actions_ci.py	85.48% <0.00%> (-3.61%)	⬇️
nf_core/utils.py	77.46% <0.00%> (-2.66%)	⬇️
nf_core/lint/__init__.py	74.07% <0.00%> (-1.56%)	⬇️
nf_core/schema.py	75.88% <0.00%> (-1.51%)	⬇️
nf_core/launch.py	63.93% <0.00%> (-0.41%)	⬇️
nf_core/modules/install.py	72.72% <0.00%> (-0.35%)	⬇️
... and 8 more

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[ErikDanielsson]

LGTM!