chore: specify verify_format=False when running pypandoc

Without this, *every* call to pypandoc.convert_text starts pandoc
three times: once to list the input formats, once to list the output
formats, and once to actually perform the conversion.

With `verify_format=False`, the first two invocations are skipped. For
some APIs this can make a huge difference to the speed of generation
(listerally about 45% in one test I've performed). We shouldn't need
to verify that the formats are supported, as we always use commonmark
and rst.

Fixes #2503.

Author: jskeet

gapic/utils/rst.py

@@ -61,6 +61,7 @@ def rst(
                 text,
                 "rst",
                 format=source_format,
+                verify_format=False,
                 extra_args=["--columns=%d" % (width - indent)],
             )
             .strip()