Command platformio package unpublish crashes

[Erriez]

• [X ] PlatformIO Core.
  If you’ve found a bug, please provide an information below.

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Configuration

Operating system: Ubuntu 18.04

PlatformIO Version (platformio --version): 5.0.0

Description of problem

The command below crashes:

platformio package unpublish [email protected]

Steps to Reproduce

1. Run the command above

Actual Results

Error: Traceback (most recent call last):
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/platformio/__main__.py", line 109, in main
    cli()  # pylint: disable=no-value-for-parameter
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/platformio/commands/__init__.py", line 44, in invoke
    return super(PlatformioCLI, self).invoke(ctx)
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/platformio/commands/package.py", line 111, in package_unpublish
    undo=undo,
  File "/home/erwin/.platformio/penv/lib/python3.6/site-packages/platformio/clients/registry.py", line 72, in unpublish_package
    path += "/" + string(version)
NameError: name 'string' is not defined

Expected Results

Package version has been successfully removed from the registry

### Additional info

Possible fix in `~/.platformio/penv/lib/python3.6/site-packages/platformio/clients/registry.py` line 71:

Change `path += "/" + version` to: `path += "/" + str(version)`