Make version check in gdb_rust_pretty_printing.py more compatible.

Some versions of Python don't support the `major` field on the
object returned by `sys.version_info`.

Author: michaelwoerister

src/etc/gdb_rust_pretty_printing.py

@@ -16,7 +16,7 @@
 # We want a version of `range` which doesn't allocate an intermediate list,
 # specifically it should use a lazy iterator. In Python 2 this was `xrange`, but
 # if we're running with Python 3 then we need to use `range` instead.
-if sys.version_info.major >= 3:
+if sys.version_info[0] >= 3:
     xrange = range
 
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