MAINT: Fixes for prospective Python 3.10 and 4.0

[hugovk]

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but whichever it is, it will probably happen in 2020 (when Python 3.9 reaches beta).

There's some places in the codebase that slice the sys.version string, assuming the string is 3 characters long. This sort of thing will break on Python 3.10:

# in python3.10 this will report as '3.1' (should be '3.10')
python_version = sys.version[0:3]

# correct way to do this
python_version = '{}.{}'.format(*sys.version_info)

And also some places that would run Python 2 code on Python 4, such as:

# in python4 this will report as `False` (and suddenly run python2 code!)
is_py3 = sys.version_info[0] == 3

# correct way to do this
is_py3 = sys.version_info[0] >= 3

Found using https://github.com/asottile/flake8-2020:

$ pip install -U flake8-2020
...
$ flake8 --select YTT
./tools/swig/test/testFarray.py:18:41: YTT101: `sys.version[:...]` referenced (python3.10), use `sys.version_info`
./tools/npy_tempita/compat3.py:8:15: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`
./numpy/distutils/command/build.py:41:54: YTT101: `sys.version[:...]` referenced (python3.10), use `sys.version_info`
./numpy/distutils/command/build_src.py:93:58: YTT101: `sys.version[:...]` referenced (python3.10), use `sys.version_info`
./numpy/core/setup.py:467:16: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`
./numpy/testing/_private/parameterized.py:48:7: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`

[charris]

Thanks @hugovk .