rose.config: ban square braces in section names

Section names are no longer allowed to use the open and close square
brace characters.
rose.config: improve pylint and pep8 scores
Merge pull request #1359 from matthewrmshin/config-disallow-square-braces-section

rose.config: ban square braces in section names
Rephrase *cycle time* to *cycle point*

Fix tests for cylc 6.
When checking whether a suite is running or not, allow reasons to
contain all lines of the `pgrep` command.
Fix `date_format` in `rose.date`.

Simply return `TimePoint` object if format string is `None`.
Fix `rose_prune` test
Improve stability of tests