Package 'rsa' requires a different Python: 2.7.18 not in '>=3.5, <4'

[jgimenez]

Steps to reproduce:

• checkout master
• docker build -t google/import-mailbox-to-gmail .
• Build stops with #8 9.284 ERROR: Package 'rsa' requires a different Python: 2.7.18 not in '>=3.5, <4'

My suspicion is that some of the dependencies have dropped support for Python 2. Any way to specify a version of the required libraries or run with Python 3?

[eesheesh]

#65 should have fixed this.