Scapy may be used as third party library by other projects, themselves
managed under git. This leads to surprising results such as this:

    $ git describe
    awesomeproject-v5.22-1-ga1316614c290
    $ virtualenv env
    ...
    $ . env/bin/activate
    (env)$ pip install scapy==2.3.3
    ...
    (env)$ cat env/lib/python2.7/site-packages/scapy/VERSION
    2.3.3
    (env)$ which scapy
    /home/.../env/bin/scapy
    (env)$ scapy
    ...
    Welcome to Scapy (awesomeproject-v5.22.dev1)
    >>>

Scapy's version is wrongly set to the current project's version.

When trying to determine scapy's version from git, make sure that the
code is executed from the root of a git repo. If not, read the version
from the scapy/VERSION file which has been generated when packaging
scapy source archive.

Fixes: 4f71027f ("enhance version management")
Signed-off-by: Robin Jarry <robin.jarry@6wind.com>