python3-packages: port from python-packages

This is largely done by suffixing "python" or "py" with "3".  The
README.md file is also copied here and we intend to maintain it
independently from its python2 counterpart.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

lang/python/python3-packages/README.md

@@ -0,0 +1,72 @@
+This package allows users to package python3 modules without creating package
+Makefiles for each individual module and their dependencies.  It provides a
+way making packaging python3 packages faster and may also facilitate the process
+of developing Makefiles for new python3 packages
+
+This is a raw DEVEL only package.  Using it may entails a lot of implementation
+details and you may need to resolve target dependencies and package details on
+your own
+
+- Third party python3 packages may depend on features not included in e.g.
+  python3-light
+- Some python3 modules may require host install of another module to progress,
+  e.g. target cryptography requires host cffi
+- Some python3 modules have external C library dependencies, e.g. pyOpenSSL
+  requires openssl libs
+- Some packages may have an autoconf configure script whose arguments we
+  cannot control with pip and has to be passed on (hacked) by overriding some
+  environment variables
+
+## How it works
+
+1. Install host modules required for building target modules
+2. Install each target module to separate directories
+3. Install another copy of modules for cleanup purposes to make list of
+   installed files to be removed from target modules installed in step 2
+
+Why should it be so
+
+1. Installing target cryptography requires host installation of cffi module
+2. cryptography requires setuptools and pip will install its own copy with
+   --ignore-installed.  When PACKAGE_python3-setuptools is also selected, opkg
+   will complain of data file clashes if it was not removed here.
+
+Pip will handle dependency requirements of python3 modules, but external
+dependencies like c libraries has to be prepared by the build system.  The
+issue is that there is currently no way to express such dependencies, thus may
+cause build failure, e.g. pycrypto requires the presence of libgmp to build
+successfully.
+
+## Tips
+
+If something goes wrong, we can add additional arguments to pip command
+line to check the detailed build process.  Some useful arguments may be
+
+- -v, for verbose output.  Repeat this option if the current level of
+  verbosity is not enough
+- --no-clean, for preserving pip build dir on build failure
+
+## Examples
+
+tornado (python-only module)
+
+	CONFIG_PACKAGE_python3-packages=y
+	CONFIG_PACKAGE_python3-packages-list="tornado==6.0.2"
+
+cryptography (requires installation of host modules and cleanup on target modules)
+
+	CONFIG_PACKAGE_python3-packages=y
+	CONFIG_PACKAGE_python3-packages-list-host="cffi"
+	CONFIG_PACKAGE_python3-packages-list="cryptography"
+	CONFIG_PACKAGE_python3-packages-list-cleanup="setuptools"
+
+pycrypto 2.7a1 (python module with autoconf configure script; depends on
+libgmp; broken wmmintrin.h).  2.6.1 does not work because of a flaw in
+the setup.py hardcoding host include directory
+
+	CONFIG_PACKAGE_libgmp=y
+	CONFIG_PACKAGE_python3-packages=y
+	CONFIG_PACKAGE_python3-packages-list="https://github.com/dlitz/pycrypto/archive/v2.7a1.tar.gz"
+	CONFIG_PACKAGE_python3-packages-envs="ac_cv_header_wmmintrin_h=no build_alias=$(GNU_HOST_NAME) host_alias=$(GNU_TARGET_NAME) target_alias=$(GNU_TARGET_NAME)"
+	CONFIG_PACKAGE_python3-packages-extra-deps="libgmp.so.10"
+