[xeddsa] fixed shared library copying

[goffi-contrib]

The copy of the build shared library was not working anymore after the
update of python2 build chain. This patch fixes it by using
self.ctx.get_site_packages_dir().

[AndreMiras]

Thanks for antoher pull request!
Same here I'd like to investigate myself why it's failing to build in the CI before merging:

�[1m[INFO]�[0m:    �[90m->�[0m running cp _crypto_sign.so /home/user/.local/share/python-...(and 61 more)�[0m
Exception in thread background thread for pid 7794:
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/home/user/venv/lib/python3.6/site-packages/sh.py", line 1540, in wrap
    fn(*args, **kwargs)
  File "/home/user/venv/lib/python3.6/site-packages/sh.py", line 2459, in background_thread
    handle_exit_code(exit_code)
  File "/home/user/venv/lib/python3.6/site-packages/sh.py", line 2157, in fn
    return self.command.handle_command_exit_code(exit_code)
  File "/home/user/venv/lib/python3.6/site-packages/sh.py", line 815, in handle_command_exit_code
    raise exc
sh.ErrorReturnCode_1: 

  RAN: /bin/cp _crypto_sign.so /home/user/.local/share/python-for-android/build/python-installs/bdisttest_python3_googlendk

  STDOUT:
/bin/cp: cannot stat '_crypto_sign.so': No such file or directory

[AndreMiras]

It still seems to be failing with that same _crypto_sign.so error. I haven't tried to reproduce it locally yet.
If this works for you locally then we may have an issue with our Docker env.
Let me know if you want me to take a look

[goffi-contrib]

@AndreMiras yes it does work for me locally. Would be nice if you could have a look indeed, thanks :)

[AndreMiras]

OK I've played with it in Docker.

(venv) user@f58394473c86:~/testapps$ find ~/ -name "*crypto_sign*.so*"
/home/user/.local/share/python-for-android/build/other_builds/xeddsa-python3/armeabi-v7a__ndk_target_21/xeddsa/ref10/bin/libcrypto_sign_dynamic.so
/home/user/.local/share/python-for-android/build/other_builds/xeddsa-python3/armeabi-v7a__ndk_target_21/xeddsa/_crypto_sign.cpython-37m-x86_64-linux-gnu.so
(venv) user@f58394473c86:~/testapps$

So the library actually is _crypto_sign.cpython-37m-x86_64-linux-gnu.so on Ubuntu. So I guess we should cp with a wildcard. I'll give it a try locally, update the PR and merge if it builds

[AndreMiras]

Conditional recipe build is green https://travis-ci.org/kivy/python-for-android/jobs/512985553
Relevant part of the log: copy ./_crypto_sign.cpython-37m-x86_64-linux-gnu.so
Merging, thanks again for the contrib!