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>

README.md

@@ -25,7 +25,7 @@ the [Build System Setup](https://openwrt.org/docs/guide-developer/build-system/i
 documentation.
 
 ```
-gcc-6+ binutils bzip2 flex python3 perl make4.1+ find grep diff unzip gawk getopt
+gcc-6+ binutils bzip2 flex python3.6+ perl make4.1+ find grep diff unzip gawk getopt
 subversion libz-dev libc-dev rsync which
 ```
 

include/prereq-build.mk

@@ -141,21 +141,19 @@ $(eval $(call SetupHostCommand,perl,Please install Perl 5.x, \
 
 $(eval $(call CleanupPython2))
 
-$(eval $(call SetupHostCommand,python,Please install Python >= 3.5, \
+$(eval $(call SetupHostCommand,python,Please install Python >= 3.6, \
 	python3.9 -V 2>&1 | grep 'Python 3', \
 	python3.8 -V 2>&1 | grep 'Python 3', \
 	python3.7 -V 2>&1 | grep 'Python 3', \
 	python3.6 -V 2>&1 | grep 'Python 3', \
-	python3.5 -V 2>&1 | grep 'Python 3', \
-	python3 -V 2>&1 | grep -E 'Python 3\.[5-9]\.?'))
+	python3 -V 2>&1 | grep -E 'Python 3\.[6-9]\.?'))
 
-$(eval $(call SetupHostCommand,python3,Please install Python >= 3.5, \
+$(eval $(call SetupHostCommand,python3,Please install Python >= 3.6, \
 	python3.9 -V 2>&1 | grep 'Python 3', \
 	python3.8 -V 2>&1 | grep 'Python 3', \
 	python3.7 -V 2>&1 | grep 'Python 3', \
 	python3.6 -V 2>&1 | grep 'Python 3', \
-	python3.5 -V 2>&1 | grep 'Python 3', \
-	python3 -V 2>&1 | grep -E 'Python 3\.[5-9]\.?'))
+	python3 -V 2>&1 | grep -E 'Python 3\.[6-9]\.?'))
 
 $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
 	git --exec-path | xargs -I % -- grep -q -- --recursive %/git-submodule))