build: prereq: drop support for Python 3.5

build: prereq: drop support for Python 3.5

The final Python 3.5 release was 3.5.10 in September 2020 [0].
This release series is now End-of-Life (EOL).

The only LTS distribution that still only ships Python 3.5 is Ubuntu 16.04,
which will be EOL in April 2021 [1].

The meson build system bumped their python requirement to 3.6 for the 0.57.0
release. This patch ensures that OpenWrt can update meson while still
relying on the host python.

[0] https://www.python.org/dev/peps/pep-0478/#id4
[1] https://ubuntu.com/about/release-cycle
[2] https://mesonbuild.com/Release-notes-for-0-57-0.html#minimum-required-python-version-updated-to-36

Signed-off-by: Andre Heider <a.heider@gmail.com>
[python3.6+ in README.md]
Signed-off-by: Petr Štetiar <ynezz@true.cz>