add pending deprecation notice for python 3.6

docs/markdown/snippets/about_minimum_python_version.md

@@ -0,0 +1,13 @@
+## Python 3.6 support will be dropped in the next release
+
+The final [Python 3.6 release was 3.6.15 in September](https://www.python.org/dev/peps/pep-0494/#lifespan).
+This release series is now End-of-Life (EOL). The only LTS distribution that
+still ships Python 3.5 as the default Python is Ubuntu 18.04, which has Python
+3.8 available as well.
+
+Python 3.7 has various features that we find useful such as future annotations,
+the importlib.resources module, and dataclasses.
+
+As a result, we will begin requiring Python 3.7 or newer in Meson 0.62, which
+is the next release. Starting with Meson 0.61, we now print a `NOTICE:` when
+a `meson` command is run on Python 3.6 to inform users about this.

mesonbuild/mesonmain.py

@@ -117,6 +117,7 @@ class CommandLineParser:
         return 0
 
     def run(self, args):
+        pending_python_deprecation_notice = False
         # If first arg is not a known command, assume user wants to run the setup
         # command.
         known_commands = list(self.commands.keys()) + ['-h', '--help']
@@ -130,10 +131,17 @@ class CommandLineParser:
             args = args[1:]
         else:
             parser = self.parser
+            command = None
 
         args = mesonlib.expand_arguments(args)
         options = parser.parse_args(args)
 
+        if command is None:
+            command = options.command
+
+        if command in ('setup', 'compile', 'test', 'install') and sys.version_info < (3, 7):
+            pending_python_deprecation_notice = True
+
         try:
             return options.run_func(options)
         except MesonException as e:
@@ -156,6 +164,9 @@ class CommandLineParser:
             mlog.exception(e)
             return 2
         finally:
+            if pending_python_deprecation_notice:
+                mlog.notice('You are using Python 3.6 which is EOL. Starting with v0.62.0, '
+                            'Meson will require Python 3.7 or newer', fatal=False)
             mlog.shutdown()
 
 def run_script_command(script_name, script_args):