Revert "Update pycairo requirement"

[soerface]

This reverts commit 17558a4.

By using ">=" in requirements, the package can get broken anytime. This has happened - on linux, 1.18.1 will be installed, resulting in this exception when using manim:

Traceback (most recent call last):
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/flask/cli.py", line 236, in locate_app
    __import__(module_name)
  File "/home/soeren/Projects/manim-gallery/src/app.py", line 9, in <module>
    from examples import hello_world
  File "/home/soeren/Projects/manim-gallery/src/examples/hello_world.py", line 1, in <module>
    from manimlib.imports import *
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/manimlib/__init__.py", line 3, in <module>
    import manimlib.extract_scene
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/manimlib/extract_scene.py", line 9, in <module>
    from manimlib.scene.scene import Scene
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/manimlib/scene/scene.py", line 11, in <module>
    from manimlib.camera.camera import Camera
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/manimlib/camera/camera.py", line 9, in <module>
    import cairo
  File "/home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/cairo/__init__.py", line 1, in <module>
    from ._cairo import *  # noqa: F401,F403
ImportError: /home/soeren/anaconda3/envs/manim-gallery/lib/python3.7/site-packages/cairo/_cairo.cpython-37m-x86_64-linux-gnu.so: undefined symbol: cairo_svg_surface_set_document_unit

[eulertour]

Should be fixed by #571.

[eulertour]

I ended up installing manim with pycairo 1.18.1 in an empty docker container and didn't run into this. It seems like the problem occurs when you have two different versions of pycairo installed at once [1]. What did you do when you ran into the error?

[soerface]

I had a fresh virtual environment (conda) when it happened, and previously only installed manim in other envs, not globally. Strange issue, I'll see if I can reproduce it properly :/

[sandikodev]

me who have some issued as same as you, fixed by #835