Cannot get utf-8 text mail to show correctly

[Orionsg]

I am using Python 3 to send mail to the server:

import	traceback
import	smtplib
from	email.mime.text		import	MIMEText
from	email.mime.multipart 	import	MIMEMultipart

def sendmail(s_from : str, s_to : str, s_srv : str, s_txt='', s_html='', s_subj='', i_port=2525, s_login='', s_pw=''):

	if not (s_from and '@' in s_from):
		raise ValueError(f'No or invalid email receiver: {s_from}')
	if not (s_to and '@' in s_to):
		raise ValueError(f'No or invalid email sender: {s_to}')
	if not s_srv:
		raise ValueError('SMTP server not specified')
	if not (s_subj or s_txt or s_html):
		raise ValueError('Either subject, text or html must be specified')
	if s_login and not s_pw or s_pw and not s_login:
		raise ValueError('Login and password must either both be specified or not at all')

	msg 			=	MIMEMultipart('alternative')
	msg['Subject']		=	s_subj
	msg['From']		=	s_from
	msg['To']		=	s_to

	if s_txt:
		mime_txt	=	MIMEText(s_txt, 'plain', 'utf-8')
		msg.attach(mime_txt)
	if s_html:
		mime_html	=	MIMEText(s_html, 'html', 'utf-8')
		msg.attach(mime_html)

	smtp_srv		=	smtplib.SMTP(s_srv, i_port)
	if s_login:
		smtp_srv.login(s_login, s_pw)
	
	smtp_srv.sendmail(s_from, s_to, msg.as_string() )

def main():

	try:
		sendmail(
			s_from	=	'[email protected]',
			s_to	=	'[email protected]',
			s_srv	=	'localhost',
			s_txt	=	'Pure text body',
			s_html	=	'<h3>HTML Header body</h3><b>Normal body text</b>',
			s_subj	=	'This is subject text'
			)

	except:
		li_s_err = str(traceback.format_exc()).split('\n')
		for s_err in li_s_err:
			print(s_err)
			
if __name__ == '__main__':
	main()

If I remove the , 'utf-8 parameter in the two calls to MIMEText, the body of the email is displayed correctly.
Otherwise, it is displayed in mailslurper as PGgzPkhUTUwgSGVhZGVyIGJvZHk8L2gzPjxiPk5vcm1hbCBib2R5IHRleHQ8L2I+

Am I using the email.mime or smtplib modules wrongly, or is there a problem with mailslurper?