Crash when using prerelease option

```
pyp2rpm --prerelease azure-common
Traceback (most recent call last):
File "/usr/bin/pyp2rpm", line 33, in
sys.exit(load_entry_point('pyp2rpm==3.3.5', 'console_scripts', 'pyp2rpm')())
File "/home/yves/.local/lib/python3.8/site-packages/click/core.py", line 1137, in __call__
return self.main(*args, **kwargs)
File "/home/yves/.local/lib/python3.8/site-packages/click/core.py", line 1062, in main
rv = self.invoke(ctx)
File "/home/yves/.local/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/home/yves/.local/lib/python3.8/site-packages/click/core.py", line 763, in invoke
return __callback(*args, **kwargs)
File "/usr/lib/python3.8/site-packages/pyp2rpm/bin.py", line 215, in main
converted = convertor.convert()
File "/usr/lib/python3.8/site-packages/pyp2rpm/convertor.py", line 135, in convert
local_file = self.getter.get()
File "/usr/lib/python3.8/site-packages/pyp2rpm/convertor.py", line 195, in getter
self._getter = package_getters.PypiDownloader(
File "/usr/lib/python3.8/site-packages/pyp2rpm/package_getters.py", line 142, in __init__
self.version = version or self.versions[0]
TypeError: 'dict_keys' object is not subscriptable
```
Note that the package doesn't exist on pypi.org.
```
pyp2rpm azure-common
azure-common package has no sources on PyPI, Please ask the maintainer to upload sources.
```
Version: pyp2rpm-3.3.5-1.mga8