Define Python supported versions as strings instead of float

[humitos]

In #8328 (comment) we realized that we are using floats to set the version of Python (3.5, 3.8, 3.10) which doesn't make sense and breaks due to 3.10 is 3.1.

This is the setting involved here:

readthedocs.org/readthedocs/settings/base.py

Lines 452 to 500 in e5fe9ac

	DOCKER_IMAGE_SETTINGS = {
	# A large number of users still have this pinned in their config file.
	# We must have documented it at some point.
	'readthedocs/build:2.0': {
	'python': {
	'supported_versions': [2, 2.7, 3, 3.5],
	'default_version': {
	2: 2.7,
	3: 3.5,
	},
	},
	},
	'readthedocs/build:4.0': {
	'python': {
	'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7],
	'default_version': {
	2: 2.7,
	3: 3.7,
	},
	},
	},
	'readthedocs/build:5.0': {
	'python': {
	'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7, 'pypy3.5'],
	'default_version': {
	2: 2.7,
	3: 3.7,
	},
	},
	},
	'readthedocs/build:6.0': {
	'python': {
	'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7, 3.8, 'pypy3.5'],
	'default_version': {
	2: 2.7,
	3: 3.7,
	},
	},
	},
	'readthedocs/build:7.0': {
	'python': {
	'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7, 3.8, 3.9, 'pypy3.5'],
	'default_version': {
	2: 2.7,
	3: 3.7,
	},
	},
	},
	}

We need to,

• define those Python versions as string
• convert the Python version defined in YAML file to string
• update tests to work with versions as string
• update documentation to mention that YAML python.version uses string
• demotivate the usage of float in YAML

There may be more changes required here, but I think they will appear as we are making the other changes. Feel free to add them to this list.

[stsewd]

This is already done