Change default downloader from aria2c to requests

This is to reduce the amount of required dependencies by not strictly requiring aria2c out of the box. You can always change the downloader back to aria2c in the config.

devine/core/config.py

@@ -50,7 +50,7 @@ class Config:
                 continue
             setattr(self.directories, name, Path(path).expanduser())
 
-        self.downloader = kwargs.get("downloader") or "aria2c"
+        self.downloader = kwargs.get("downloader") or "requests"
 
         self.filenames = self._Filenames()
         for name, filename in (kwargs.get("filenames") or {}).items():