The `jsonschema` module switched the packaging system to Hatch since
version 4.6.0 (python-jsonschema/jsonschema#957).  The build process for
`jsonschema` now requires the `hatchling` and `hatch-vcs` modules, but
the used Nixpkgs snapshot does not include Nix packages for these
modules; adding these modules to the `[tool.poetry.dev-dependencies]`
section makes them available for the (upcoming) poetry2nix override
without the need to bump the Nixpkgs snapshot.

However, simply adding `hatchling` and `hatch-vcs` to dependencies
produces some more dependency issues:

  1) The `pytest` module in the used Nixpkgs snapshot is old and has a
     dependency for the `pluggy` module with an upper bound for the
     version, which creates a version conflict with the dependency of
     the `hatchling` module.  Upgrading the `pytest` module to the
     current version fixes this compatibility issue.

  2) The `tomli` module, which is in the dependency tree of `hatchling`,
     requires a newer `flit-core` module than found in the used Nixpkgs
     snapshot.  Again, upgrading the `flit-core` module to the current
     version fixes the compatibility issue.

When the Nixpkgs snapshot gets bumped in the future, this commit should
probably be reverted (and `poetry.lock` should be regenerated to remove
the unneeded dependencies).

The `jsonschema` module switched the packaging system to Hatch since
version 4.6.0 (python-jsonschema/jsonschema#957).  The build process for
`jsonschema` now requires the `hatchling` and `hatch-vcs` modules, which
were made available to poetry2nix by the previous commits; add these
modules to `nativeBuildInput` to fix the build.