Drop support for Python < 2.6

[latk]

The code base needs to be compatible across a wide range of Python versions. But that can only go so far. The user guide mentions these supported versions:

CPython 2.7, 3.4, and 3.5, and PyPy 2.7 and 3.5.

It is also likely that some users are stuck on 2.6, so we shouldn't intentionally break 2.6 compatibility (at least not without a good reason).

But support of earlier versions has disproportianate cost and little value. This PR removes any code that is unreachable under 2.6 or later.

As an additional change, the unit tests now use decorators to annotate the test classes.

[hugovk]

Here's the pip installs for gcovr from PyPI for the last month (via pypinfo --percent --pip --markdown gcovr pyversion)

python_version	percent	download_count
2.7	65.3%	3,716
3.5	16.7%	949
3.4	14.8%	842
2.6	2.1%	118
3.6	1.1%	65

[hugovk]

You could also add python_requires to setup.py to help pip. For example, see: https://stackoverflow.com/a/44660547/724176

Also, it would be useful to add version classifiers to setup.py (https://pypi.python.org/pypi?%3Aaction=list_classifiers).

And there's a few other bits that can be removed:

• gcovr/admin/virtualenv.py

  Lines 120 to 121 in 16c81cc

  	if minver <= 3:
  	REQUIRED_MODULES.extend(['sets', '__future__'])

• gcovr/admin/virtualenv_1.7.py

  Lines 84 to 85 in 16c81cc

  	if minver <= 3:
  	REQUIRED_MODULES.extend(['sets', '__future__'])

• gcovr/admin/virtualenv_1.7.py

  Lines 25 to 29 in 16c81cc

  	if sys.version_info <= (2, 3):
  	print('ERROR: %s' % sys.exc_info()[1])
  	print('ERROR: this script requires Python 2.4 or greater; or at least the subprocess module.')
  	print('If you copy subprocess.py from a newer version of Python this script will probably work')
  	sys.exit(101)

(There's probably some old Python 3.x stuff that can go from there too, is only 3.4+ supported?)

[latk]

Thank you @hugovk for these statistics – they demonstrate that this change will not hurt users.

And thank you too for the suggestions re classifiers and python_requires, I'll look into those shortly.

The remaining unneeded version checks are constrained to the vendored virtualenv scripts. This will be resolved by deleting the scripts completely in #190.