Syntax error in copyright check in 2018-patches

The copyright check seems to be failing multiple MRs to 2018-patches across different projects:

$ curl -o check_copyright.py "https://gitlab.cern.ch/lhcb-core/LbDevTools/raw/master/LbDevTools/SourceTools.py?inline=false"
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 28070  100 28070    0     0   119k      0 --:--:-- --:--:-- --:--:--  119k
$ python check_copyright.py origin/2018-patches
  File "check_copyright.py", line 55
    ENCODING_DECLARATION = re.compile(rb"^[ \t\f]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)")
                                                                                       ^
SyntaxError: invalid syntax
Cleaning up project directory and file based variables

See examples from Phys, Rec and Analysis.

This problem seems to have been introduced in !222 (diffs).