Show a warning message about deprecated support for Python 2 and Python 3.5

HISTORY.rst

@@ -16,6 +16,7 @@ PlatformIO Core 5
 - Updated analysis tools:
     * `Cppcheck <https://docs.platformio.org/page/plus/check-tools/cppcheck.html>`__ v2.3 with improved C++ parser and several new MISRA rules
     * `PVS-Studio <https://docs.platformio.org/page/plus/check-tools/pvs-studio.html>`__ v7.11 with new diagnostics and updated mass suppression mechanism
+- Show a warning message about deprecated support for Python 2 and Python 3.5
 - Do not provide "intelliSenseMode" option when generating configuration for VSCode C/C++ extension
 - Fixed a "git-sh-setup: file not found" error when installing project dependencies from Git VCS (`issue #3740 <https://github.com/platformio/platformio-core/issues/3740>`_)
 
@@ -139,7 +140,8 @@ Please check `Migration guide from 4.x to 5.0 <https://docs.platformio.org/page/
   - Display system-wide information using a new `pio system info <https://docs.platformio.org/page/core/userguide/system/cmd_info.html>`__ command (`issue #3521 <https://github.com/platformio/platformio-core/issues/3521>`_)
   - Remove unused data using a new `pio system prune <https://docs.platformio.org/page/core/userguide/system/cmd_prune.html>`__ command (`issue #3522 <https://github.com/platformio/platformio-core/issues/3522>`_)
   - Show ignored project environments only in the verbose mode (`issue #3641 <https://github.com/platformio/platformio-core/issues/3641>`_)
-  - Do not escape compiler arguments in VSCode template on Windows.
+  - Do not escape compiler arguments in VSCode template on Windows
+  - Drop support for Python 2 and 3.5.
 
 .. _release_notes_4:
 

platformio/compat.py

@@ -62,10 +62,11 @@ def ci_strings_are_equal(a, b):
 
 
 def ensure_python3(raise_exception=True):
-    if not raise_exception or not PY2:
-        return not PY2
+    compatible = sys.version_info >= (3, 6)
+    if not raise_exception or compatible:
+        return compatible
     raise UserSideException(
-        "Python 3.5 or later is required for this operation. \n"
+        "Python 3.6 or later is required for this operation. \n"
         "Please install the latest Python 3 and reinstall PlatformIO Core using "
         "installation script:\n"
         "https://docs.platformio.org/page/core/installation.html"

platformio/maintenance.py

@@ -27,6 +27,7 @@ from platformio.commands.lib.command import CTX_META_STORAGE_DIRS_KEY
 from platformio.commands.lib.command import lib_update as cmd_lib_update
 from platformio.commands.platform import platform_update as cmd_platform_update
 from platformio.commands.upgrade import get_latest_version
+from platformio.compat import ensure_python3
 from platformio.package.manager.core import update_core_packages
 from platformio.package.manager.library import LibraryPackageManager
 from platformio.package.manager.platform import PlatformPackageManager
@@ -43,8 +44,25 @@ def on_platformio_start(ctx, force, caller):
     set_caller(caller)
     telemetry.on_command()
 
-    if not PlatformioCLI.in_silence():
-        after_upgrade(ctx)
+    if PlatformioCLI.in_silence():
+        return
+
+    after_upgrade(ctx)
+
+    if not ensure_python3(raise_exception=False):
+        click.secho(
+            """
+Python 2 and Python 3.5 are not compatible with PlatformIO Core 5.0.
+Please check the migration guide on how to fix this warning message:
+""",
+            fg="yellow",
+        )
+        click.secho(
+            "https://docs.platformio.org/en/latest/core/migration.html"
+            "#drop-support-for-python-2-and-3-5",
+            fg="blue",
+        )
+        click.echo("")
 
 
 def on_platformio_end(ctx, result):  # pylint: disable=unused-argument