This also removes PKG_BUILD_PARALLEL:=0 that was added for packages that
use HOST_PYTHON3_PACKAGE_BUILD_DEPENDS.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

In hash-checking mode[1], pip will verify downloaded package archives
(source tarballs in our case) against known SHA256 hashes before
installing the packages.

As a consequence, this requires the use of requirements files[2] and
pinning packages to known versions.

The syntax for package Makefiles has changed slightly;
HOST_PYTHON3_PACKAGE_BUILD_DEPENDS no longer accepts requirement
specifiers like "foo>=1.0", only requirements file names (which are the
same as package names in the most common case).

This also updates affected packages, in particular:

* python-zipp: "setuptools_scm[toml]" has been split into
  "setuptools-scm toml" to reuse the requirements file for
  setuptools-scm (the extra depends installed by "setuptools_scm[toml]"
  is toml).

* python-pycparser: This previously used ply 3.10, whereas the
  requirements file will now install 3.11.

[1]: https://pip.pypa.io/en/stable/reference/pip_install/#hash-checking-mode
[2]: https://pip.pypa.io/en/stable/user_guide/#requirements-files

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

This adds PKG_BUILD_PARALLEL:=0 to packages that depend on host Python
packages (HOST_PYTHON3_PACKAGE_BUILD_DEPENDS), because installing
packages with multiple concurrent pip processes can lead to errors or
unexpected results[1].

This also:

* Move HOST_PYTHON3_PACKAGE_BUILD_DEPENDS definitions to before
  python3-package.mk is included

* Update Python folder readme to include PKG_BUILD_PARALLEL:=0

[1]: https://github.com/pypa/pip/issues/2361

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

These packages were in the PR stage when the cleanup occurred and so
still had VARIANT:=python3.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

Signed-off-by: Javier Marcet <javier@marcet.info>

Signed-off-by: Javier Marcet <javier@marcet.info>