Replace <br> with space in BeautifulSoap output

Using replace() on the html before parsing:

from bs4 import BeautifulSoup

html = '''<h1 class="para-title">A quick brown fox jumps over<br>the lazy dog
<span>some stuff here</span></h1>'''

html = html.replace("<br>", " ")
soup = BeautifulSoup(html, 'html.parser')
title_box = soup.find('h1', attrs={'class': 'para-title'})
title = title_box.get_text().strip()
print (title)

OUTPUT:

A quick brown fox jumps over the lazy dog
some stuff here

EDIT:

For the part OP mentioned in the comments below;

html = '''<div class="description">Planet Nine was initially proposed to explain the clustering of orbits
Of Planet Nine's other effects, one was unexpected, the perpendicular orbits, and the other two were found after further analysis. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four.
</div>'''

from bs4 import BeautifulSoup

html = html.replace("\n", ". ")
soup = BeautifulSoup(html, 'html.parser')
div_box = soup.find('div', attrs={'class': 'description'})
divText= div_box.get_text().strip()
print (divText)

OUTPUT:

Planet Nine was initially proposed to explain the clustering of orbits. Of Planet Nine's other effects, one was unexpected, the perpendicular orbits, and the other two were found after further analysis. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four..

How about using the .get_text() with the separator parameter?

from bs4 import BeautifulSoup

page = '''<h1 class="para-title">A quick brown fox jumps over<br>the lazy dog
<span>some stuff here</span></h1>'''


soup = BeautifulSoup(page, 'html.parser')
title_box = soup.find('h1', attrs={'class': 'para-title'})
title = title_box.get_text(separator=" ").strip()
print (title)   

Output:

print (title)
A quick brown fox jumps over the lazy dog
 some stuff here