Update constants.py

[srnlorenz]

This resolves the following issue:

$ python setup.py install
Traceback (most recent call last):
  File "/builds/esgf/environments/esgf-prepare/setup.py", line 7, in <module>
    from esgprep.utils.constants import VERSION
  File "/builds/esgf/environments/esgf-prepare/esgprep/utils/constants.py", line 15
    VERSION_DATE = datetime(year=2019, month=04, day=15).strftime("%Y-%d-%m")
                                             ^
SyntaxError: leading zeros in decimal integer literals are not permitted; use an 0o prefix for octal integers

[srnlorenz]

Hello ESGF team, would it be possible to merge this PR?