CodingGuidelines: specify Python 2.7 is the oldest version

In 0b4396f068 (git-p4: make python2.7 the oldest supported version,
2019-12-13), git-p4 was updated to only support 2.7 and newer. Since
Python 2.6 is pretty much ancient history, update CodingGuidelines to
show that 2.7 is the oldest version supported.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -483,16 +483,11 @@ For Python scripts:
 
  - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 
- - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
+ - As a minimum, we aim to be compatible with Python 2.7.
 
  - Where required libraries do not restrict us to Python 2, we try to
    also be compatible with Python 3.1 and later.
 
- - When you must differentiate between Unicode literals and byte string
-   literals, it is OK to use the 'b' prefix.  Even though the Python
-   documentation for version 2.6 does not mention this prefix, it has
-   been supported since version 2.6.0.
-
 Error Messages
 
  - Do not end error messages with a full stop.