requirements.txt: Avoid depending on PyYAML<6

Some distros like Archlinux already ship with PyYAML 6, which makes it
impossible to build jenkins-job-builder.

I tried manually dropping the "PyYAML < 6" dependency and I could update
some jenkins jobs without problem. Let's not block newer versions from
now on in order o avoid blocking downstream.

Related: https://storyboard.openstack.org/#!/story/2009723
Related: https://aur.archlinux.org/packages/python-jenkins-job-builder#comment-860221
Change-Id: I2308020ed226615afd2652ff74d6766b5ecfab43

requirements.txt

@@ -2,7 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 six>=1.9.0 # MIT
-PyYAML>=3.10.0,<6 # MIT
+PyYAML>=3.10.0 # MIT
 pbr>=1.8 # Apache-2.0
 stevedore>=1.17.1,<2; python_version < '3.0'  # Apache-2.0
 stevedore>=1.17.1; python_version >= '3.0'  # Apache-2.0