Support EmPy version 4

[hjoliver]

A forum post has revealed that EmPy 3 doesn't play well with scheduler daemonization:

https://cylc.discourse.group/t/empy-import-shell-environment-variables/985/1

If I simply add #!EmPy to a flow.cylc, the workflow runs fine but:

• I get a traceback at start-up
• stdout does not end up in the log
• stdout still appears in the terminal

Something to do with EmPy replacing sys.stdout

--- Logging error ---
Traceback (most recent call last):
  File "/home/oliverh/cylc/cylc-flow/cylc/flow/loggingutil.py", line 167, in emit
    self.do_rollover()
  File "/home/oliverh/cylc/cylc-flow/cylc/flow/loggingutil.py", line 234, in do_rollover
    os.dup2(self.stream.fileno(), sys.stdout.fileno())
AttributeError: 'ProxyFile' object has no attribute 'fileno'

The good news: EmPy 4.1 works fine, but it requires a small change to our code.

Anyone aware of any reason not to require EmPy > 4 ?

Check List

• [ ] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [ ] Contains logically grouped changes (else tidy your branch by rebase).
• [ ] Does not contain off-topic changes (use other PRs for other changes).
• [ ] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [ ] Tests are included (or explain why tests are not needed).
• [ ] Changelog entry included if this is a change that can affect users
• [ ] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [ ] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.