TYP: Add a version stub to allow mypy to run without building

[effigies]

Thanks to @Factral in #1208 (comment) I realized that we could probably make the contribution process a little nicer to people not wanting to pip install -e or hatch build --hooks-only just to make the pre-commit hooks run cleanly.

This adds a _version.pyi that mypy will respect without even checking whether _version.py is present.

The biggest potential risk is setuptools-scm changing the contents of _version.py in the future, but that seems low and a thing we can deal with if it comes up.

No need to backport unless we get contributors to a maintenance branch.

Verifying:

~/Projects/nipy 
❯ git clone nibabel/.git nibabel-mypy-test
Cloning into 'nibabel-mypy-test'...
done.
~/Projects/nipy 
❯ cd nibabel-mypy-test 
nibabel-mypy-test on typ/version_stub 
❯ pre-commit install
pre-commit installed at .git/hooks/pre-commit
nibabel-mypy-test on typ/version_stub 
❯ pre-commit run --all
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...............................................................Passed
check json...............................................................Passed
check toml...............................................................Passed
check for added large files..............................................Passed
check for case conflicts.................................................Passed
check for merge conflicts................................................Passed
check vcs permalinks.....................................................Passed
blue.....................................................................Passed
isort....................................................................Passed
flake8...................................................................Passed
mypy.....................................................................Passed

Reverting to master:

❯ git checkout master
Branch 'master' set up to track remote branch 'master' from 'origin'.
Switched to a new branch 'master'
nibabel-mypy-test on master 
❯ pre-commit run --all               
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...............................................................Passed
check json...............................................................Passed
check toml...............................................................Passed
check for added large files..............................................Passed
check for case conflicts.................................................Passed
check for merge conflicts................................................Passed
check vcs permalinks.....................................................Passed
blue.....................................................................Passed
isort....................................................................Passed
flake8...................................................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

nibabel/pkg_info.py:9: error: Cannot find implementation or library stub for module named "nibabel._version"  [import]
nibabel/pkg_info.py:9: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 107 source files)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.08 🎉

Comparison is base (3a4cc5e) 92.06% compared to head (cf43308) 92.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1210      +/-   ##
==========================================
+ Coverage   92.06%   92.14%   +0.08%     
==========================================
  Files          97       97              
  Lines       12334    12334              
  Branches     2534     2534              
==========================================
+ Hits        11355    11365      +10     
+ Misses        655      647       -8     
+ Partials      324      322       -2     

Impacted Files	Coverage Δ
nibabel/spatialimages.py	94.06% <0.00%> (+0.91%)	⬆️
nibabel/nicom/ascconv.py	90.12% <0.00%> (+2.46%)	⬆️
nibabel/testing/__init__.py	98.18% <0.00%> (+2.72%)	⬆️
nibabel/__init__.py	100.00% <0.00%> (+3.17%)	⬆️
nibabel/pydicom_compat.py	88.23% <0.00%> (+5.88%)	⬆️

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