requirements: Re-drop direct dependency on mock.
requirements: Re-drop direct dependency on mock.

This was dropped in commit 840cf4b8850c3f87b4f6c11287e2f8f98609258d
(#15091), but commit 143206795949d85545d1a76f634d23b5b780898f
(#17047) mistakenly reintroduced it.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

andersk/zulip,punchagan/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,kou/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,rht/zulip