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

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

asv/commands/update.py:                        log.warning("{}: {}".format(path, err))
asv/commands/update.py:                                log.warning("{}: should be renamed to {}".format(path, new_path))
asv/commands/update.py:                        log.warning("{}: unrecognized file name".format(path))

[asnramos]

xref #1079

[datapythonista]

Not useful anymore, and add noise to the issues list.