doc: mention Python 3.x supports

doc: mention Python 3.x supports

Commit 0b4396f068, (git-p4: make python2.7 the oldest supported version,
2019-12-13) pointed out that git-p4 uses Python 2.7-or-later features
in the code.

In addition, git-p4 gained enough support for Python 3 from
6cec21a82f, (git-p4: encode/decode communication with p4 for
python3, 2019-12-13).

Let's update our documentation to reflect that fact.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>