use more py38 features

[ThomasWaldmann]

master branch requires py39+ (soon, see #6315).
1.2-maint branch requires py38+.
1.1-maint will stay on py35+.

https://docs.python.org/3/whatsnew/3.6.html

especially:

• use blake2 from python and drop our stuff? #5634
• f-strings: https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-pep498
• secrets module: https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-pep506 - we could use that instead of os.urandom at some places. maybe have a better readable random_bytes alias for the secrets.token_bytes function?
• scrypt maybe, additionally to the currently used pbkdf2? see Using a more modern KDF than PBKDF2 #747.
• https://docs.python.org/3/whatsnew/3.6.html#datetime - see the comment in borg.constants about isoformat() being evil - that's not the case any more with timespec != 'auto'!
• https://docs.python.org/3/whatsnew/3.6.html#enum
• https://docs.python.org/3/whatsnew/3.6.html#time
• try https://docs.python.org/3/whatsnew/3.6.html#pythonmalloc-environment-variable - anything unusual/unexpected?
• try https://docs.python.org/3/whatsnew/3.6.html#tracemalloc - anything unusual/unexpected?

https://docs.python.org/3/whatsnew/3.7.html

• data classes: https://docs.python.org/3/whatsnew/3.7.html#dataclasses (but also check hynek's attrs)
• https://docs.python.org/3/library/datetime.html#datetime.datetime.fromisoformat - see borg.helpers.time.parse_timestamp- needs more testing
• https://docs.python.org/3/library/hmac.html#hmac.digest - see hmac and blake2b minor optimizations and cleanups #6408
• https://docs.python.org/3/library/os.html#os.scandir can use a FD now - we already use that, we could just remove the py_37_plus check.

https://docs.python.org/3/whatsnew/3.8.html

• https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions
• https://docs.python.org/3/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging
• https://docs.python.org/3/whatsnew/3.8.html#tarfile pax by default

[RonnyPfannschmidt]

Consider pyupgrade as helper

[ThomasWaldmann]

https://pypi.org/project/pyupgrade/

# zsh
pyupgrade --py38-plus ./**/*.py

[ThomasWaldmann]

#6382 < pyupgrade run.