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>

This removes:

* Python 3 variants (VARIANT:=python3)

* "for Python3" from package titles

* Package selection condition from package dependencies, e.g.
  +PACKAGE_python3-six:python3-light replaced with +python3-light

* "Default" package information sections, e.g.
  Package/python-six/Default removed and package details merged into
  Package/python3-six

* "(Variant for Python3)" from package descriptions

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

Signed-off-by: Jan Pavlinec <jan.pavlinec@nic.cz>