Migrates gevent to new python3 recipe

[AndreMiras]

Also adds unit tests for checking flags are being cleaned up properly.

[AndreMiras]

Depends on greelet #1603 which should be merged first

[AndreMiras]

Conditional build is failing because of permission issues in the Docker env while symlinking, see part of the trace:

  File "/home/user/pythonforandroid/recipe.py", line 979, in build_arch
    self.build_cython_components(arch)
  File "/home/user/pythonforandroid/recipes/gevent/__init__.py", line 23, in build_cython_components
    shprint(sh.ln, '-sf', link_c + '.so', link_rt + '.so')
  File "/home/user/pythonforandroid/logger.py", line 178, in shprint
    for line in output:
  File "/home/user/venv/local/lib/python2.7/site-packages/sh.py", line 863, in next
    self.wait()
  File "/home/user/venv/local/lib/python2.7/site-packages/sh.py", line 792, in wait
    self.handle_command_exit_code(exit_code)
  File "/home/user/venv/local/lib/python2.7/site-packages/sh.py", line 815, in handle_command_exit_code
    raise exc
sh.ErrorReturnCode_1: 
  RAN: /bin/ln -sf /opt/android/android-ndk-r17c/platforms/android-21/arch-arm/usr/lib/libc.so /opt/android/android-ndk-r17c/platforms/android-21/arch-arm/usr/lib/librt.so
  STDOUT:
/bin/ln: failed to create symbolic link '/opt/android/android-ndk-r17c/platforms/android-21/arch-arm/usr/lib/librt.so': Permission denied

This is very specific to our Docker setup since the NDK is installed in /opt/android and requires super privilege to add the temporary symlink.
I think we're still safe to merge nonetheless since prior version of the recipe won't even compile with recent p4a refactoring

[inclement]

++