Example doesn't work

[k3rne1pan1c]

Describe the bug
Using the example in the documentation throws an error due to a negative value used as a parameter.

To Reproduce
`
from kivy.garden.mapview import MapView, MapMarker
from kivy.app import App

class FirstKivy(App):

def build(self):

    map = MapView(zoom=13, lat=50.6394, lon=3.057)
    m1 = MapMarker(lon=50.6394, lat=3.057)  # Lille
    m2 = MapMarker(lon=-33.867, lat=151.206)

    map.add_marker(m1)
    map.add_marker(m2)

    return map

FirstKivy().run()
`

Expected behavior
Should work if you use it as an example

Logs/output

Traceback (most recent call last):
File "/home/k3rne1pan1c/Documents/projects/wherRu/main.py", line 15, in
FirstKivy().run()
File "/home/k3rne1pan1c/Documents/projects/wherRu/lib/python3.9/site-packages/kivy/app.py", line 949, in run
self._run_prepare()
File "/home/k3rne1pan1c/Documents/projects/wherRu/lib/python3.9/site-packages/kivy/app.py", line 919, in _run_prepare
root = self.build()
File "/home/k3rne1pan1c/Documents/projects/wherRu/main.py", line 12, in build
map.add_marker(m2)
File "/home/k3rne1pan1c/.kivy/garden/garden.mapview/mapview/view.py", line 464, in add_marker
layer.set_marker_position(self, marker)
File "/home/k3rne1pan1c/.kivy/garden/garden.mapview/mapview/view.py", line 247, in set_marker_position
x, y = mapview.get_window_xy_from(marker.lat, marker.lon, mapview.zoom)
File "/home/k3rne1pan1c/.kivy/garden/garden.mapview/mapview/view.py", line 361, in get_window_xy_from
y = ms.get_y(zoom, lat) - vy
File "/home/k3rne1pan1c/.kivy/garden/garden.mapview/mapview/source.py", line 94, in get_y
return ((1.0 - log(tan(lat) + 1.0 / cos(lat)) / pi) /
ValueError: math domain error

If applicable, add screenshots to help explain your problem.

Platform (please complete the following information):

• OS: Pop!_OS 21.04
• Python version: 3.9.5

[k3rne1pan1c]

https://mapview.readthedocs.io/en/latest/usage/

[k3rne1pan1c]

I figured out, that in the develop branch you fixed the kivy updates. Can you merge the develop in the master? @tshirtman