Dflit

-----------

- New projects created with :ref:`init_cmd` now declare that they require
``flit_core >=2,<4`` (:ghpull:`328`). Any projects using ``pyproject.toml``
(not ``flit.ini``) should be compatible with flit 3.x.
- Fix selecting files from a git submodule to include in an sdist
(:ghpull:`324`).
- Fix checking classifiers when no writeable cache directory is available
(:ghpull:`319`).
- Better errors when trying to install to a mis-spelled or missing Python
interpreter (:ghpull:`331`).
- Fix specifying ``--repository`` before ``upload`` (:ghpull:`322`). Passing the
option like this is deprecated, and you should now pass it after ``upload``.
- Documentation improvements (:ghpull:`327`, :ghpull:`318`, :ghpull:`314`)

-----------

- Allow underscores in package names with Python 2 (:ghpull:`305`).
- Add a ``--no-setup-py`` option to build sdists without a backwards-compatible
``setup.py`` file (:ghpull:`311`).
- Fix the generated ``setup.py`` file for packages using a ``src/`` layout
(:ghpull:`303`).
- Fix detecting when more than one file matches the module name specified
(:ghpull:`307`).
- Fix installing to a venv on Windows with the ``--python`` option
(:ghpull:`300`).
- Don't echo the command in scripts installed with ``--symlink`` or
``--pth-file`` on Windows (:ghpull:`310`).
- New ``bootstrap_dev.py`` script to set up a development installation of Flit
from the repository (:ghpull:`301`, :ghpull:`306`).

-----------

- Use compression when adding files to wheels.
- Added the :envvar:`FLIT_INSTALL_PYTHON` environment variable (:ghpull:`295`),
to configure flit to always install into a Python other than the one it's
running on.
- ``flit_core`` uses the ``intreehooks`` shim package to load its bootstrapping
backend, until a released version of pip supports the standard
``backend-path`` mechanism.

-----------

Flit 2 is a major architecture change. The ``flit_core`` package now provides
a :pep:`517` backend for building packages, while ``flit`` is a
:doc:`command line interface <cmdline>` extending that.

The build backend works on Python 2, so tools like pip should be able to install
packages built with flit from source on Python 2.
The ``flit`` command requires Python 3.5 or above.
You will need to change the build-system table in your ``pyproject.toml`` file
to look like this:

.. code-block:: toml

[build-system]
requires = ["flit_core >=2,<4"]
build-backend = "flit_core.buildapi"

Other changes include:

- Support for storing your code under a ``src/`` folder (:ghpull:`260`).
You don't need to change any configuration if you do this.
- Options to control what files are included in an sdist - see
:ref:`pyproject_toml_sdist` for the details.
- Requirements can specify a URL 'direct reference', as an alternative to a
version number, with the syntax defined in :pep:`440`:
``requests https://example.com/requests-2.22.0.tar.gz``.
- Fix the shebang of scripts installed with the ``--python`` option and the
``--symlink`` flag (:ghpull:`286`).
- Installing with ``--deps develop`` now installs normal dependencies
as well as development dependencies.
- Author email is no longer required in the metadata table (:ghpull:`289`).
- More error messages are now shown without a traceback (:ghpull:`254`)

-----------

- Fix for building sdists from a subdirectory in a Mercurial repository
(:ghpull:`233`).
- Fix for getting the docstring and version from modules defining their encoding
(:ghpull:`239`).
- Fix for installing packages with ``flit installfrom`` (:ghpull:`221`).
- Packages with requirements no longer get a spurious ``Provides-Extra: .none``
metadata entry (:ghissue:`228`).
- Better check of whether ``python-requires`` includes any Python 2 version
(:ghpull:`232`).
- Better check of home page URLs in ``flit init`` (:ghpull:`230`).
- Better error message when the description file is not found (:ghpull:`234`).
- Updated a help message to refer to ``pyproject.toml`` (:ghpull:`240`).
- Improve tests of ``flit init`` (:ghpull:`229`).

-------------

- Fix for installing packages with ``flit install``.
- Make ``requests_download`` an extra dependency, to avoid a circular build
dependency. To use ``flit installfrom``, you can install with
``pip install flit[installfrom]``. Note that the ``installfrom`` subcommand
is deprecated, as it will soon be possible to use pip to install Flit projects
directly from a VCS URL.