chore: bump pygments to 2.15.0 in .kokoro (#800)

Author: jackwotherspoon

.kokoro/requirements.txt

@@ -396,9 +396,9 @@ pycparser==2.21 \
     --hash=sha256:8ee45429555515e1f6b185e78100aea234072576aa43ab53aefcae078162fca9 \
     --hash=sha256:e644fdec12f7872f86c58ff790da456218b10f863970249516d60a5eaca77206
     # via cffi
-pygments==2.13.0 \
-    --hash=sha256:56a8508ae95f98e2b9bdf93a6be5ae3f7d8af858b43e02c5a2ff083726be40c1 \
-    --hash=sha256:f643f331ab57ba3c9d89212ee4a2dabc6e94f117cf4eefde99a0574720d14c42
+pygments==2.15.0 \
+    --hash=sha256:77a3299119af881904cd5ecd1ac6a66214b6e9bed1f2db16993b54adede64094 \
+    --hash=sha256:f7e36cffc4c517fbc252861b9a6e4644ca0e5abadf9a113c72d1358ad09b9500
     # via
     #   readme-renderer
     #   rich