tools/checkcerts.py: fix deprecated datetime.datetime.utcnow()

The replacement, datetime.datetime.now(datetime.UTC), returns a
tz-aware item, so the expiry time must also be timezone-aware.
datetime.strptime() does not handle that time format sanely.
I don't know why.  The code seems to require a gmtoffset along with
the timezone name (so what's the point of the timezone name).  Shrug.
Supply it one for GMT.  If we ever get a certificate whose expiry
date is not in GMT, it'll be a problem.

Signed-off-by: Dan Mick <dan.mick@redhat.com>

Author: dmick

tools/checkcerts.py

@@ -100,9 +100,18 @@ def main():
             errstr = f'{domain} cert error: {e}'
 
         if not certerr:
-            expire = datetime.datetime.strptime(cert['notAfter'],
-                '%b %d %H:%M:%S %Y %Z')
-            now = datetime.datetime.utcnow()
+            # apparently strptime only returns a tz-aware datetime iff it gets both
+            # a timezone name and a UTC offset.  Add an offset.  'Z' is magic for -0000.
+            #
+            # yes, this only handles GMT
+            exp=cert['notAfter'].split()
+            assert(exp[-1] == 'GMT')
+            exp.append('Z')
+            exp = ' '.join(exp)
+
+            expire = datetime.datetime.strptime(exp,
+                '%b %d %H:%M:%S %Y %Z %z')
+            now = datetime.datetime.now(datetime.UTC)
             left = expire - now
 
             errstr = f'{domain:30s} cert: {str(left).rsplit(".",1)[0]} left until it expires'