Python versions in setup.py are outdated

[BSchilperoort]

era5cli/setup.py

Lines 40 to 43 in 970226a

	'Programming Language :: Python',
	'Programming Language :: Python :: 3',
	'Programming Language :: Python :: 3.6',
	'Programming Language :: Python :: 3.7',

See the above lines in setup.py. Only the Python versions 3.6 (out of support) and 3.7 are displayed, while the CI covers 3.6 through 3.9.

[BSchilperoort]

I now see that this issue was already raised in #119.