cros: Enforce minimum Python version of 3.8

Our developer guide has stated that Python 3.8 or greater is required
since CL:4303078.  Additionally, this launcher has been warning users
of Python 3.6 and 3.7 for quite some time now.  Finally, Python 3.8 is
now the default interpeter in CrOS SDK.

I think it's safe to make 3.8 the hard minimum version.

R=vapier@chromium.org

Bug: b:187794810
Change-Id: I0d71ba916dafedd08757edc5beff539b788494bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4483617
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

cros

@@ -21,7 +21,7 @@ import sys
 # Min version of Python that we *want*.  We warn for older versions.
 MIN_PYTHON_VER_SOFT = (3, 8)
 # Min version of Python that we *require*.  We abort for older versions.
-MIN_PYTHON_VER_HARD = (3, 6)
+MIN_PYTHON_VER_HARD = (3, 8)
 
 DEPOT_TOOLS_DIR = pathlib.Path(__file__).resolve().parent