testing/mitmproxy: security upgrade to 7.0.3
fixes CVE-2021-39214
security advisory: https://github.com/mitmproxy/mitmproxy/security/advisories/GHSA-22gh-3r9q-xf38
fixes CVE-2021-39214
security advisory: https://github.com/mitmproxy/mitmproxy/security/advisories/GHSA-22gh-3r9q-xf38