Fix missing pywin32 wheel version

Version 300 does not exist.

Running:
    vpython3 -vpython-spec src/.vpython3 -vpython-tool verify

gives the error:
    failed to resolve infra/python/wheels/pywin32/windows-386_cp311_cp311@version:300 (line 97): no such tag

Change-Id: I6c42853c285892c946f5723f36649b0eaefedf17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6164608
Auto-Submit: Benjamin Joyce (Ben) <bjoyce@chromium.org>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Benjamin Joyce (Ben) <bjoyce@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404985}

.vpython3

@@ -501,7 +501,7 @@ wheel: <
 #   //third_party/blink/tools/blinkpy/web_tests/port/server_process.py
 wheel: <
   name: "infra/python/wheels/pywin32/${vpython_platform}"
-  version: "version:300"
+  version: "version:306"
   match_tag: <
     platform: "win32"
   >