Remove unsupported Python 2.4 and 2.5 code

[hugovk]

No description provided.

[coveralls]

Coverage remained the same at 24.667% when pulling 2d60560 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[coveralls]

Coverage remained the same at 24.667% when pulling 2d60560 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[coveralls]

Coverage increased (+0.04%) to 24.702% when pulling c115489 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[coveralls]

Coverage increased (+0.04%) to 24.702% when pulling 6d9e302 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[coveralls]

Coverage increased (+0.04%) to 24.702% when pulling 6d9e302 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[coveralls]

Coverage increased (+0.04%) to 24.702% when pulling 1b950b9 on hugovk:patch-1 into f2bece4 on python-diamond:master.

[hugovk]

Here's the pip installs for diamond from PyPI for the last month:

$ pypinfo --percent --pip diamond pyversion
python_version percent download_count
-------------- ------- --------------
2.7              98.0%         27,185
2.6               1.8%            509
3.6               0.1%             22
3.5               0.0%              8
3.4               0.0%              3

[hugovk]

@shortdudey123 Hello, any thoughts on this PR? Thanks!