drop compatibility for older python releases

[matysek]

• I think that support for older python releases should be droped in favor of python 3 support.
• I think that support for Python < 2.3 should be dropped.
• Other python projects dropped support for Python < 2.3 or 2.4 in favour of py3 (​scons 2.0, ​sqlalchemy, ​lxml, ​waf)

Notes:

• python 2.3 is available on RHEL4 and derivates
• source code and tests in svn works now only with python >= 2.3
  • modules platform, zipimport, optparse is in py >= 2.3
  • ctypes in python >= 2.5
• in branch waf, loader expects in python class exceptions.Warning - in python since 2.1
• pywin32 builds are available for python >= 2.3

[matysek]

sys.version_info in py >= 2.0

[matysek]

some code cleanup when dropping support for python 2.2 and older

[giovannibajo]

I think it is reasonable to drop Python 2.3 and older versions whenever we move to support Python 3.0, but I don't think we should do this earlier. There are real world users of Python 2.2 using PyInstaller? for instance, and I know at least one case of Python 2.1.

[matysek]

For 1.6 we would like to make changes for future python3 support (#356). This requires to support only python 2.4+.

[matysek]

For now the only reason to drop python 2.3 support is to use subprocess module instead os.system() for python3 preparation. It makes no sense to drop 2.3 when python3 support is not yet done.

And we could support python 2.3 even when using subprocess module.

We should find out more meaningful reasons why do drop python 2.3 support'''

[htgoebel]

Notable new features in 2.4 are:

• subprocess module
• set type is now built in
• generator expressions
• Simpler String Substitutions
• Decorators for Functions and Methods
• str.rsplit()

So it looks as if the only relevant change/enhancement is the subprocess module.

[matysek]

Latest dev version supports only Python 2.4+.