Security fixes
Upgrade Tor Browser to 9.0.7-build1 (Closes: #17539).
Upgrade tor to 0.4.2.7 (Closes: #17531).
Upgrade Thunderbird to 1:68.6.0-1~deb10u1 (MFSA-2020-10, DSA-4642).
Upgrade WebKitGTK to 2.26.4-1~deb10u2 (DSA-4641).
Build system
lint_po: avoid race conditions when checking PO files (Closes: #17359).