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>

INSTALL

@@ -165,8 +165,7 @@ Issues of note:
 	  use English. Under autoconf the configure script will do this
 	  automatically if it can't find libintl on the system.
 
-	- Python version 2.4 or later (but not 3.x, which is not
-	  supported by Perforce) is needed to use the git-p4 interface
+	- Python version 2.7 or later is needed to use the git-p4 interface
 	  to Perforce.
 
  - Some platform specific issues are dealt with Makefile rules,

Makefile

@@ -303,7 +303,7 @@ all::
 # modules, instead of the fallbacks shipped with Git.
 #
 # Define PYTHON_PATH to the path of your Python binary (often /usr/bin/python
-# but /usr/bin/python2.7 on some platforms).
+# but /usr/bin/python2.7 or /usr/bin/python3 on some platforms).
 #
 # Define NO_PYTHON if you do not want Python scripts or libraries at all.
 #