Fix GitHub Actions setup and test config

[mbrukman]

Per the most recent GitHub Actions run, fix the setup and test commands:

• remove --use-mirrors flag from the pip command as it's no longer a valid option failed pip run
• remove Python 3.3 and 3.4 from the list of versions as they are not available on macOS (failed Python 3.3 install, failed Python 3.4 install)

The list of available Python versions shows Python 3.5 and later are provided.

Minor changes:

• add Ubuntu 18.04 LTS to the list of OS versions in case folks still use it, since it's still supported and may be in use
• explicitly specify the x64 architecture to clarify that we're running in 64-bit mode (also the default), as opposed to x86 which would run in 32-bit mode

Closes https://github.com/rossant/ipycache/issues/52

[rossant]

Perhaps old versions of Python like 2.7, 3.5, 3.6, may be dropped?

[mbrukman]

Perhaps old versions of Python like 2.7, 3.5, 3.6, may be dropped?

Done.

[mbrukman]

@rossant — since we've removed testing with Python 2.7, should we also remove special-casing for Python 2 (see below), or should we bring back the tests for Python 2.7 to keep backwards-compatibility? I know Python 2.x was EOL as of Jan 2020, but maybe some folks somewhere are still depending on using it (somehow)?

https://github.com/rossant/ipycache/blob/ebefe10d69d2e519060b4767c5778ab808fb488e/ipycache.py#L28-L52

[rossant]

I think it's fine to drop Python 2.x support. Maybe add a tag before doing it to be on the safe side..