Travis CI: Add Python 3.9 release candidate 1

[cclauss]

• Python 3.9 in allow_failures mode until release on Oct. 5th https://www.python.org/download/pre-releases
• Now that pip has a real dependancy resolver, feed it all requirements in one command
• Fix Travis build config validation issues: os, sudo

Note: Python 3.5 goes EOL on Sunday.

[MerlijnWajer]

@JoeLametta - Should we deprecate Python 3.5 ahead of the EOL?

[JoeLametta]

@JoeLametta - Should we deprecate Python 3.5 ahead of the EOL?

It seems that python 3.5 will reach its EOL in 3 days anyway:

Branch	Schedule	Status	First release	End-of-life	Release manager
master	PEP 619	features	2021-10-04	TBD	Pablo Galindo Salgado
3.9	PEP 596	bugfix	2020-10-05	TBD	Łukasz Langa
3.8	PEP 569	bugfix	2019-10-14	2024-10	Łukasz Langa
3.7	PEP 537	security	2018-06-27	2023-06-27	Ned Deily
3.6	PEP 494	security	2016-12-23	2021-12-23	Ned Deily
3.5	PEP 478	security	2015-09-13	2020-09-13	Larry Hastings

I think we can mark it as deprecated on 2020-09-13 while adding a python 3.5 job as allow_failures in Travis CI (just to know if we break 3.5 compatibility unknowingly).

[JoeLametta]

@cclauss Thanks for the pull request: it looks good to me.