Use inspect.getsourcelines to get code

[judahrand]

This function was not used previously due to a bug in how it cached source
code. This bug was fixed in Python 3.5 which is now EOL. This also allows
us to detect the actual line in source code of a lambda reducing possible
collisions.

See https://bugs.python.org/issue1218234

[ogrisel]

Let's wait for #1253 to be merged and then I will push a merge commit to this PR to re-run the CI.

[ogrisel]

I haven't looked at the details on what those 2 tests do but they fail on this PR while they pass on master:

=================================== FAILURES ===================================
_________________________ test_special_source_encoding _________________________

    def test_special_source_encoding():
        from joblib.test.test_func_inspect_special_encoding import big5_f
        func_code, source_file, first_line = get_func_code(big5_f)
>       assert first_line == 5
E       assert -1 == 5
E         +-1
E         -5

big5_f     = <function big5_f at 0x0000000006323600>
first_line = -1
func_code  = '-8379453629057454882'
source_file = '/home/vsts/work/1/s/joblib/test/test_func_inspect_special_encoding.py'

joblib/test/test_func_inspect.py:277: AssertionError
__________________________ test_func_code_consistency __________________________

    def test_func_code_consistency():
        from joblib.parallel import Parallel, delayed
        codes = Parallel(n_jobs=2)(delayed(_get_code)() for _ in range(5))
>       assert len(set(codes)) == 1
E       assert 2 == 1
E         +2
E         -1

Parallel   = <class 'joblib.parallel.Parallel'>
codes      = ['9209186506343720391', '3446341992231549869', '3446341992231549869', '3446341992231549869', '3446341992231549869']
delayed    = <function delayed at 0x00007f50a25e1740>