Replace f-strings (Python >=3.6) by a direct format call

[eldipa]

This pr propose to replace all the f-strings by a call to format (but there are a lot of different ways to replace f-strings to something compatible with Python 3.x), one of the may blockers to run termgraph in Python 3.x (see #28 )

I ran the unit tests with this:

$ python -m unittest -q tests/test_termgraph.py

And all except 3 tests pass:

• test_init_args
• test_calendar_heatmap_without_start_date_prints_correctly
• test_check_data_stacked_with_no_color_returns_correct_result

However they don't look to be related with the fact that I ran them in Python 3.5.

[mkaz]

I rebased and merged. Thanks!