Order of exceptions in retries.if_exception_type is unstable

[busunkim96]

See https://github.com/googleapis/python-firestore/pull/197/files/ for an example.

The exceptions do not seem to get consistently sorted in alphabetical order.

The fluctuating order shows up in the three files with this clause.

gapic-generator-python/gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/transports/base.py.j2

Lines 91 to 97 in b2e5274

	predicate=retries.if_exception_type(
	{%- filter sort_lines %}
	{%- for ex in method.retry.retryable_exceptions %}
	exceptions.{{ ex.__name__ }},
	{%- endfor %}
	{%- endfilter %}
	),