[server] drop system_source_cnx for a simpler internal API system_source_cnx() can be replaced with source_cnx(). If system_source_cnx() is called with dbms_system_base=False, it is strictly equivalent to calling source_cnx() with the same source, privileges and interactivity. If system_source_cnx() is called with dbms_system_base=True, it is equivalent to calling source_cnx() with the system database and the same source, privileges and interactivity. No use of the default value for 'special_privs' was found in the code.

### Requirements for tests in various cubicweb/**/test directories. ###

## shared by several test folders
docutils
webtest

## cubicweb/test
Pygments
pycryptodomex
#fyzz XXX pip install fails

## cubicweb/devtools/test
flake8

## cubicweb/hooks/test
psycopg2-binary

## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth
repoze.lru

## cubicweb/sobject/test