fix encoding issues when using umlauts in from/to headers

[MrTango]

No description provided.

[petschki]

I'd use plone.base.utils.safe_bytes instead.

And the import at the top should be changed to plone.base.utils.safe_text

[mauritsvanrees]

LGTM, thanks.

Requested changes have been made.

[mauritsvanrees]

I rebased on master and force-pushed.

[mauritsvanrees]

Lots of test failures like this:

AttributeError: 'bytes' object has no attribute 'encode'

[mauritsvanrees]

@MrTango I don't know why no tests show up here. But locally lots of tests fail like this:

Error in test testMultiRecipientOverrideByString (collective.easyform.tests.testMailer.TestFunctions.testMultiRecipientOverrideByString)
Traceback (most recent call last):
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/unittest/case.py", line 57, in testPartExecutor
    yield
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/unittest/case.py", line 623, in run
    self._callTestMethod(testMethod)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/unittest/case.py", line 579, in _callTestMethod
    if method() is not None:
  File "/Users/maurits/community/collective.easyform/src/collective/easyform/tests/testMailer.py", line 380, in testMultiRecipientOverrideByString
    mailer.onSuccess(data, request)
  File "/Users/maurits/community/collective.easyform/src/collective/easyform/actions.py", line 553, in onSuccess
    mailtext = self.get_mail_text(fields, request, context)
  File "/Users/maurits/community/collective.easyform/src/collective/easyform/actions.py", line 548, in get_mail_text
    return outer.as_string()
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/message.py", line 188, in as_string
    g.flatten(self, unixfrom=unixfrom)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/generator.py", line 117, in flatten
    self._write(msg)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/generator.py", line 200, in _write
    self._write_headers(msg)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/generator.py", line 227, in _write_headers
    folded = self.policy.fold(h, v)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/_policybase.py", line 334, in fold
    return self._fold(name, value, sanitize=True)
  File "/Users/maurits/.pyenv/versions/3.11.11/lib/python3.11/email/_policybase.py", line 377, in _fold
    parts.append(h.encode(linesep=self.linesep, maxlinelen=maxlinelen))
AttributeError: 'bytes' object has no attribute 'encode'

I wonder if on Python 3 these headers need to be passed in as strings (text) anyway now...