Support python3 as interpreter for internal pylama

[mfonville]

With python2 EOL and distro's not packaging python2 anymore (or deprecating it) and atom also finally dropping python2 as a requirement (today with 1.47.0) and python3 taking its place as default. linter-pylama fails if python is not installed, and if it does not refer to python2. Because if it points to python3 instead it gives the error:

Traceback (most recent call last): File "/home/xxx/.atom/packages/linter-pylama/bin/pylama.py", line 12, in from pylama.main import shell File "/home/xxx/.atom/packages/linter-pylama/bin/pylama/main.py", line 10 from .async import check_async ^ SyntaxError: invalid syntax

Could linter-pylama please support having python3 as interpreter for the internal pylama? either for python or with python3

[mfonville]

(which seems to at least merge PR #112