fix error brought from wrong python-magic deps

[czxvan]

The python-magic==0.4.28 has a bug that has been disscussed in ahupp/python-magic#313, which could cause this error:

root@65cd61496027:/workspaces/firmafl-repro# python3 -c "import magic"
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python3.5/dist-packages/magic/init.py", line 234, in
libmagic = loader.load_lib()
File "/usr/local/lib/python3.5/dist-packages/magic/loader.py", line 77, in load_lib
raise ImportError("python-magic: failed to find libmagic. Check your installation")
ImportError: python-magic: failed to find libmagic. Check your installation

We can just install python-magic from pypi whose version is 0.4.27, which is good.