Gamepad not working in Linux
**Describe the bug**
Checking for Gamepad inputs using `pyxel.btn(pyxel.GAMEPAD_1_LEFT)` for example does not return true on Linux.  Have tried both a Xbox One and a PS4 controller connected via USB.  Confirmed working in OS.

**To Reproduce**
Steps to reproduce the behavior:
1. Be running Linux
2. Check inputs on GAMEPAD_1 or GAMEPAD_2
3. Do something to indicate buttons were pressed

**Expected behavior**
`btn` expected to return true when `[key]` is `GAMEPAD_1_LEFT` for example.

**Desktop (please complete the following information):**
 - OS: Linux - Manjaro, Kernel 5.9.3-1, Cinnamon 4.6.7
- Python 3.8.6
- Pyxel 1.4.3

**Additional context**
Tried both `GAMEPAD_1` and `GAMEPAD_2`.  Building and running both from PyCharm and from stand-alone executable
