Move project config to pyproject.toml, remove BINDER support

[sappelhoff]

• related to maint: linters, CIs, packaging metadata #1174

other to dos to be completed in future PRs.

I am also removing BINDER here

we'd have to keep a requirements file for it, and I don't think BINDER is used all that much for browsing the mne-bids examples.

to do

• update release protocol (mention of req files, mention of binder)
• remove BINDER environment.yml file from gh-pages branch

[sappelhoff]

• CI issues due to: BUG: Move defusedxml to optional dependencies mne-python#12264

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aaa9a3a) 97.63% compared to head (1eaa36f) 97.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1202      +/-   ##
==========================================
- Coverage   97.63%   97.58%   -0.06%     
==========================================
  Files          40       40              
  Lines        8684     8684              
==========================================
- Hits         8479     8474       -5     
- Misses        205      210       +5     

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