Recommended Cython version is 0.29.21

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Bump host Cython version as well.
Add note near PKG_VERSION to remember to periodically update it.

Signed-off-by: Alexandru Ardelean <ardeleanalex@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>

Essentially, this is a re-spin from
  https://github.com/openwrt/packages/pull/9797/

But a really trimmed down version.
Only the Py3 variant is added now, which makes the Makefile really small
now.

Cython is needed on the host, to cythonize some files.

The package needs targets with hard-float enabled. This is because on some
soft-float targets floating-point exception constants aren't defined.
We can define some dummy values, but that ends up being a bit too much
work.
So, for that, the package depends on HAS_FPU or KERNEL_MIPS_FPU_EMULATOR.
This way, numpy should be buildable on hard-float, or for MIPS on the FPU
emulator being compiled in.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>