1.21.0: pep417 build requires `pre-commit` which is not needed

[kloczek]

+ /usr/bin/python3 -sBm build -w --no-isolation
* Getting dependencies for wheel...
running egg_info
creating jupyter_server.egg-info
writing jupyter_server.egg-info/PKG-INFO
writing dependency_links to jupyter_server.egg-info/dependency_links.txt
writing entry points to jupyter_server.egg-info/entry_points.txt
writing requirements to jupyter_server.egg-info/requires.txt
writing top-level names to jupyter_server.egg-info/top_level.txt
writing manifest file 'jupyter_server.egg-info/SOURCES.txt'
reading manifest file 'jupyter_server.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files found matching 'docs/#*'
no previously-included directories found matching 'docs/build'
no previously-included directories found matching 'docs/gh-pages'
no previously-included directories found matching 'docs/dist'
warning: no previously-included files matching '*~' found anywhere in distribution
warning: no previously-included files matching '*.pyo' found anywhere in distribution
no previously-included directories found matching '.git'
no previously-included directories found matching '**/.ipynb_checkpoints'
no previously-included directories found matching '**/.pytest_cache'
no previously-included directories found matching '**/.coverage'
no previously-included directories found matching '**/.pytest_cache'
adding license file 'COPYING.md'
writing manifest file 'jupyter_server.egg-info/SOURCES.txt'

ERROR Missing dependencies:
        pre-commit

[kloczek]

pre-commit is not needed as well in test suite.

[blink1073]

pre-commit is now a test-only dependency.