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>