MAINT: remove PY35 workarounds

[oesteban]

[effigies]

Let's drop it altogether.

[mgxd]

Some outstanding optimizations to address down the line (taken from #487 (comment))

$ find . -name "*.py" -exec grep -H "35" {} \;
./niworkflows/reports/core.py:        # PY35: Sorted config dict for consistent behavior
./niworkflows/reports/core.py:                # PY35: Coerce to str to pacify os.* functions that don't take Paths until 3.6
./niworkflows/utils/misc.py:    for f in os.scandir(path):  # switch to context manager when py35 dropped