Replace wget with requests library. ### [Codacy](https://app.codacy.com/gh/XCSoar/mapgen/commit?cid=693315890) detected an issue: #### Message: `Consider possible security implications associated with subprocess module.` #### Currently on: + **Commit**: 12275f75f012349ba0f9b464dd68947259dfc319 + **File**: [lib/xcsoar/mapgen/downloader.py](https://github.com/XCSoar/mapgen/blob/12275f75f012349ba0f9b464dd68947259dfc319/lib/xcsoar/mapgen/downloader.py) + **LineNum**: [5](https://github.com/XCSoar/mapgen/blob/12275f75f012349ba0f9b464dd68947259dfc319/lib/xcsoar/mapgen/downloader.py#L5)