test.get_opts fails on minion if minion_sign_messages: True

Description of Issue

If minion_sign_messages: True is set in the minion, running salt 'minion_id' test.get_opts fails:

2019-12-29 20:57:18,671 [salt.minion      :1482][INFO    ][1844] User sudo_user Executing command test.get_opts with jid 20191229205717044139
2019-12-29 20:57:21,328 [salt.minion      :1151][INFO    ][2680] Creating minion process manager
2019-12-29 20:57:21,404 [salt.minion      :1609][INFO    ][2680] Starting a new job 20191229205717044139 with PID 2680
2019-12-29 20:57:21,467 [salt.minion      :1937][INFO    ][2680] Returning information for job: 20191229205717044139
2019-12-29 20:57:21,467 [salt.utils.process:754 ][ERROR   ][2680] An un-handled exception from the multiprocessing process 'SignalHandlingMultiprocessingProcess-1' was caught:
Traceback (most recent call last):
  File "c:\salt\bin\lib\site-packages\salt\utils\process.py", line 747, in run
    return super(MultiprocessingProcess, self).run()
  File "c:\salt\bin\lib\multiprocessing\process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "c:\salt\bin\lib\site-packages\salt\minion.py", line 1594, in _target
    run_func(minion_instance, opts, data)
  File "c:\salt\bin\lib\site-packages\salt\minion.py", line 1589, in run_func
    return Minion._thread_return(minion_instance, opts, data)
  File "c:\salt\bin\lib\site-packages\salt\minion.py", line 1776, in _thread_return
    timeout=minion_instance._return_retry_timer()
  File "c:\salt\bin\lib\site-packages\salt\minion.py", line 1997, in _return_pub
    ret_val = self._send_req_sync(load, timeout=timeout)
  File "c:\salt\bin\lib\site-packages\salt\minion.py", line 1406, in _send_req_sync
    sig = salt.crypt.sign_message(minion_privkey_path, salt.serializers.msgpack.serialize(load))
  File "c:\salt\bin\lib\site-packages\salt\serializers\msgpack.py", line 65, in _serialize
    raise SerializationError(error)
SerializationError: can't serialize datetime.datetime(2019, 12, 29, 21, 57, 16, 781000)

Setup

Minion's config file:
minion_sign_messages: True

Steps to Reproduce Issue

On master:
salt 'minion_id' test.get_opts

Versions Report

Master:

Salt Version:
           Salt: 2019.2.2
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: 2.5.3
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.9.4
        libgit2: 0.24.5
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.8
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: 0.24.2
         Python: 2.7.13 (default, Sep 26 2018, 18:42:22)
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.4.3
            ZMQ: 4.2.1
 
System Versions:
           dist: debian 9.11 
         locale: UTF-8
        machine: x86_64
        release: 4.9.0-11-amd64
         system: Linux
        version: debian 9.11

Minion:

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: 1.12.2
       cherrypy: 17.4.1
       dateutil: 2.8.0
      docker-py: Not Installed
          gitdb: 2.0.6
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10.1
        libgit2: Not Installed
        libnacl: 1.6.1
       M2Crypto: Not Installed
           Mako: 1.0.7
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.19
       pycrypto: Not Installed
   pycryptodome: 3.8.1
         pygit2: Not Installed
         Python: 2.7.15 (v2.7.15:ca079a3ea3, Apr 30 2018, 16:30:26) [MSC v.1500 64 bit (AMD64)]
   python-gnupg: 0.4.4
         PyYAML: 3.13
          PyZMQ: 18.0.1
           RAET: Not Installed
          smmap: 2.0.5
        timelib: 0.2.4
        Tornado: 4.5.3
            ZMQ: 4.3.1

System Versions:
           dist:
         locale: cp1252
        machine: AMD64
        release: 10
         system: Windows
        version: 10 10.0.17763  Multiprocessor Free

Reproduce salt 'minion_id' test.get_opts with minion_sign_messages: True. Start in salt/minion.py at _send_req_sync and follow the call into salt/serializers/msgpack.py _serialize; done means the command completes without the datetime serialization exception and returns the requested options.