[MAINT] update gitblame ignore and contributing

[Remi-Gau]

In my opinion this is the last PR for the formatting automation
and so this closes #2528

Changes proposed in this pull request:

• update contributing to say that we now use: black and isort
• update gitblame ignore

[Remi-Gau]

I will tackle #3653 after this: will probably ask some people with better gitfu than me at the OHBM hackaton to help me figure out the git blame ignore is not working

[github-actions]

👋 @Remi-Gau Thanks for creating a PR!

Until this PR is ready for review, you can include the [WIP] tag in its title, or leave it as a github draft.

Please make sure it is compliant with our contributing guidelines. In particular, be sure it checks the boxes listed below.

• PR has an interpretable title.
• PR links to Github issue with mention Closes #XXXX (see our documentation on PR structure)
• Code is PEP8-compliant (see our documentation on coding style)
• Changelog or what's new entry in doc/changes/latest.rst (see our documentation on PR structure)

For new features:

• There is at least one unit test per new function / class (see our documentation on testing)
• The new feature is demoed in at least one relevant example.

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

We will review it as quick as possible, feel free to ping us with questions if needed.

[Remi-Gau]

Should I also mention the following in this table?

https://nilearn.github.io/stable/development.html#contribution-guidelines

• style your code using black
• sort import
• use f strings

[ymzayek]

Sure makes sense

[codecov]

Codecov Report

Merging #3842 (2445ed4) into main (d38e49d) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3842      +/-   ##
==========================================
- Coverage   91.57%   91.56%   -0.01%     
==========================================
  Files         133      133              
  Lines       15619    15645      +26     
  Branches     3248     3256       +8     
==========================================
+ Hits        14303    14326      +23     
  Misses        770      770              
- Partials      546      549       +3     

Flag	Coverage Δ
macos-latest_3.10	91.49% <ø> (?)
macos-latest_3.11	91.49% <ø> (-0.01%)	⬇️
macos-latest_3.8	91.45% <ø> (-0.01%)	⬇️
macos-latest_3.9	91.45% <ø> (-0.01%)	⬇️
ubuntu-latest_3.10	91.49% <ø> (-0.01%)	⬇️
ubuntu-latest_3.11	91.49% <ø> (-0.01%)	⬇️
ubuntu-latest_3.8	91.45% <ø> (?)
ubuntu-latest_3.9	91.45% <ø> (-0.01%)	⬇️
windows-latest_3.10	91.43% <ø> (-0.01%)	⬇️
windows-latest_3.11	91.43% <ø> (?)
windows-latest_3.8	91.39% <ø> (-0.01%)	⬇️
windows-latest_3.9	91.39% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 4 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[bthirion]

LGTM, thx !

[ymzayek]

LGTM!