Package dependencies are too strict

**Describe the bug**
https://github.com/cs01/gdbgui/blob/master/requirements.txt is linked in `setup.py` as package dependencies. All of these are locked to one and only version. This might be useful for testing purposes but it causes very likely dependency hell and version clashing with other packages installed.

Shouldn't be https://github.com/cs01/gdbgui/blob/master/requirements.in the one which should be used as the package dependency?

**To Reproduce**
1. Have anything which is incompatible with anything from `requirements.txt`, e.g. with `click==8.1.7`.
2. `pip gdbgui==0.15.2.0` cannot be installed because of clashing dependencies.