Pin chardet to 3.0.4

Chardet 3.0.4 was last modified in 2017, and the next release of chardet 4.0.0
was released only recently (Dec 2020).  As many other pip packages still depend
on 3.0.4 we need to explicitly pin that here to avoid conflicts later at LCM
docker build time.

Change-Id: I2a1ee0d61e641c0dc3f240345ce47f0d13d8fb22
Signed-off-by: beierlm <mark.beierl@canonical.com>