Wouldn't this return False if r.name is 'Python 3.10'?

Probably! But that's just what the existing code does. I was going to open a separate issue to go through the whole repo to fix all similar version checks prior to 3.10.0 a year from now. Do you have any idea if there are other such version checks?

Yep!

>>> 'Python 3.10' >= 'Python 3.9'
False

A quick grep doesn't show any more of this pattern:

$ git grep -E "[23]\..* [<>][=]? "
$ git grep -E " [<>][=]? .*[23]\."
static/sass/_layout.scss:        & > .column { width: 33.3333%; }
templates/downloads/os_list.html:                        {% if os.slug == 'windows' and r.name >= 'Python 3.5' %}

Running https://github.com/asottile/flake8-2020/ doesn't find anything:

$ pip install -qU flake8-2020 && flake8 --select YTT
$

Still worth a more thorough review.

If anyone wants to come up with a proper version test, please feel free to do so! My immediate concern is the imminent 3.9.0 release.

Agreed, this PR looks good for 3.9.0, and we've a year to fix it properly for 3.10.0.

Something like this?

>>> from packaging import version  # pip install packaging
>>> def parse_version(version_string):
...     return version.parse(version_string.split()[1])
...
>>> parse_version("Python 3.10") >= parse_version("Python 3.9")
True
>>> parse_version("Python 3.10") <= parse_version("Python 3.9")
False
>>> parse_version("Python 3.10") == parse_version("Python 3.10")
True
>>>

Oh, that looks like a good approach. I forgot about packaging.

And rather than the .split()[1], looks like we could use r.get_version():

def parse_version(version_string):
     return version.parse(version_string)