[Sprint] Use f-strings instead .format() function on asv/config.py

[LucyJimenez]

xref #909

Since we support Python > 3.7 now, we can make use of f-strings, to make code more readable.

As an example, the next asv code:
machine_env_name = "{}/{}".format(machine, env_name)
or the next, which is equivalent:
machine_env_name = "{0}/{1}".format(machine, env_name)
Could be written as:
machine_env_name = f"{machine}/{env_name}"
(note the f before the string quotes)

The next command returns the list of strings to replace:
grep -R --exclude-dir="build" --exclude="*.css" ".format(" asv/config.py

For this file, you will find the following issues to solve:

asv/config.py:        self.pythons = ["{0[0]}.{0[1]}".format(sys.version_info)]
asv/config.py:            raise util.UserError("Config file {0} not found.".format(path))
asv/config.py:                "No repo specified in {0} config file.".format(path))