This instructs tools like pip that meson requires python 3.5 or greater,
so if one tries to install on an older version pip will fall back to an
older version of meson.

This won't fix 0.45 since it's already in the wild (unless someone wants
to delete and re-upload the version on pypi). But it should fix future
versions.

Retain the hand rolled version check, but update it to enforce our
actual requirements.

This ensures the installed scripts will have updated shebangs.

Fixes: https://github.com/mesonbuild/meson/issues/1677

./setup.py:17:1: F401 'os' imported but unused
import os
^
./setup.py:37:1: F401 'stat.ST_MODE' imported but unused
from stat import ST_MODE
^
./run_tests.py:17:1: F401 'os' imported but unused
import subprocess, sys, os
^
./run_tests.py:18:1: F401 'shutil' imported but unused
import shutil
^
./run_unittests.py:23:1: F401 'mesonbuild.dependencies.Qt5Dependency' imported but unused
from mesonbuild.dependencies import PkgConfigDependency, Qt5Dependency
^
./mesonbuild/build.py:15:1: F401 '.coredata' imported but unused
from . import coredata
^
./mesonbuild/interpreter.py:32:1: F401 'subprocess' imported but unused
import os, sys, subprocess, shutil, uuid, re
^
./mesonbuild/interpreter.py:32:1: F401 're' imported but unused
import os, sys, subprocess, shutil, uuid, re
^
./mesonbuild/dependencies.py:23:1: F401 'subprocess' imported but unused
import os, stat, glob, subprocess, shutil
^
./mesonbuild/mesonlib.py:17:1: F401 'sys' imported but unused
import platform, subprocess, operator, os, shutil, re, sys
^
./mesonbuild/modules/qt5.py:15:1: F401 'subprocess' imported but unused
import os, subprocess
^
./mesonbuild/modules/pkgconfig.py:15:1: F401 '..coredata' imported but unused
from .. import coredata, build
^
./mesonbuild/scripts/scanbuild.py:15:1: F401 'sys' imported but unused
import sys, os
^
./mesonbuild/scripts/meson_exe.py:20:1: F401 'subprocess' imported but unused
import subprocess
^
./mesonbuild/scripts/meson_exe.py:22:1: F401 '..mesonlib.MesonException' imported but unused
from ..mesonlib import MesonException, Popen_safe
^
./mesonbuild/scripts/symbolextractor.py:23:1: F401 'subprocess' imported but unused
import os, sys, subprocess
^
./mesonbuild/scripts/symbolextractor.py:25:1: F401 '..mesonlib.MesonException' imported but unused
from ..mesonlib import MesonException, Popen_safe
^
./mesonbuild/scripts/meson_install.py:19:1: F401 '..mesonlib.MesonException' imported but unused
from ..mesonlib import MesonException, Popen_safe
^
./mesonbuild/scripts/yelphelper.py:15:1: F401 'sys' imported but unused
import sys, os
^
./mesonbuild/scripts/yelphelper.py:20:1: F401 '..mesonlib.MesonException' imported but unused
from ..mesonlib import MesonException
^
./mesonbuild/backend/vs2010backend.py:17:1: F401 're' imported but unused
import re
^
./test cases/vala/8 generated sources/src/copy_file.py:3:1: F401 'os' imported but unused
import os
^
./test cases/common/107 postconf/postconf.py:3:1: F401 'sys' imported but unused
import sys, os
^
./test cases/common/129 object only target/obj_generator.py:5:1: F401 'shutil' imported but unused
import sys, shutil, subprocess
^
./test cases/common/57 custom target chain/usetarget/subcomp.py:3:1: F401 'os' imported but unused
import sys, os
^
./test cases/common/95 dep fallback/subprojects/boblib/genbob.py:3:1: F401 'os' imported but unused
import os
^
./test cases/common/98 gen extra/srcgen.py:4:1: F401 'os' imported but unused
import os
^
./test cases/common/113 generatorcustom/gen.py:3:1: F401 'os' imported but unused
import sys, os
^
./test cases/common/113 generatorcustom/catter.py:3:1: F401 'os' imported but unused
import sys, os
^
./test cases/common/59 object generator/obj_generator.py:5:1: F401 'shutil' imported but unused
import sys, shutil, subprocess
^

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>