Cleanup useless sys.version_info checks

[tristanlatr]

We just drop Python 3.8 but I did not cleaned-up all legacy code.

pydoctor/pydoctor/test/test_astbuilder.py (these are inside test cases source, so it's not dead code)
2171,8: if sys.version_info > (3,7):
2252,20: if sys.version_info > (3,0):
2254,22: elif sys.version_info > (2,6):
2293,20: if sys.version_info < (3,7):
2314,8: if sys.version_info > (3,0):

pydoctor/pydoctor/test/test_twisted_python_deprecate.py
197,8: if sys.version_info>(3.8):

pydoctor/pydoctor/astbuilder.py
29,4: if sys.version_info >= (3,8):

pydoctor/pydoctor/astutils.py
235,8: if sys.version_info >= (3,8):
492,25: _string_lineno_is_end = sys.version_info < (3,8)
500,8: if sys.version_info >= (3,8):
520,8: if sys.version_info >= (3,8):
536,8: if sys.version_info >= (3,8):
567,8: if sys.version_info >= (3,8):

pydoctor/pydoctor/model.py
58,25: _string_lineno_is_end = sys.version_info < (3,8) \