Bug 1713818: Require Python 3.6+ to run Mach r=ahal

This will both:
* Ensure that developers aren't accidentally using Mach with Python 2
  in the year 2021.
* Confirm that CI tasks aren't still using Python 2 with Mach.
* Remove support for Python 3.5, which is EOL'd.

Differential Revision: https://phabricator.services.mozilla.com/D116484

build/mach_bootstrap.py

@@ -193,9 +193,9 @@ def bootstrap(topsrcdir):
     # Ensure we are running Python 2.7 or 3.5+. We put this check here so we
     # generate a user-friendly error message rather than a cryptic stack trace
     # on module import.
-    major, minor = sys.version_info[:2]
-    if (major == 2 and minor < 7) or (major == 3 and minor < 5):
-        print("Python 2.7 or Python 3.5+ is required to run mach.")
+    major = sys.version_info[:2][0]
+    if sys.version_info < (3, 6):
+        print("Python 3.6+ is required to run mach.")
         print("You are running Python", platform.python_version())
         sys.exit(1)
 

python/mozboot/mozboot/bootstrap.py

@@ -287,13 +287,6 @@ class Bootstrapper(object):
         subprocess.check_call((sys.executable, mach_binary, "install-moz-phab"))
 
     def bootstrap(self, settings):
-        if sys.version_info[0] < 3:
-            print(
-                "This script must be run with Python 3. \n"
-                'Try "python3 bootstrap.py".'
-            )
-            sys.exit(1)
-
         if self.choice is None:
             # Like ['1. Firefox for Desktop', '2. Firefox for Android Artifact Mode', ...].
             labels = [