[pkg] various fixes - add missing files to MANIFEST - exclude doc/changes from dh_installchangelogs (it's a directory, which dh_installchangelogs doesn't expect) - fix doc build in debian package

[doc] fix broken symlinks

[pkg] update 3.21.0 release date

[doc/3.21] mention the postgresql-only state of the upgrade script

[datafeed] also catch EnvironmentError when trying to load the cwclientlib config file means the config file has not been found, so fall back to the old implementation instead of crashing.

[schema] Make syntax compatible with print_function yams seems to exec() us sometimes (instead of __import__()), and yams.reader uses print_function nowadays.

[pkg] 3.21.0

[test] use FakeFile instead of File in test_fulltextindex File.data is no longer fulltextindexed.

i18n update

[serverctl] fix schema-diff command It hadn't been ported away from dbapi.

[serverct] call server_{maintenance,shutdown} hooks around source-sync command

[serverctl] call server_{maintenance,shutdown} hooks around add-source

Update hgignore

delete unmaintained clone_deps script Closes #2517580.

merge 3.20.9 into 3.21 Fix conflict in unittest_postgres.py by changing sources configuration in PostgresTimeoutConfiguration.__init__ so it happens *after* setUpModule().