switch to python3.8

[meganung]

clean_install37 failing because a dependency bump now requires python 3.8 so we switch to python 3.8:

• Update the circle CI config file to use python 3.8 and remove 3.7
• add tomli<2.0.0 requirement to get around black error:
  ran into another error: cleaninstall_38 is failing due to a black issue (the fix hasn’t been in a release)

[stephenroller]

Amazing. TYVM!

[klshuster]

do we not have to worry about the multiple character, single-hyphen cmd line args? e.g. -vtim? I thought I remembered this being an issue with 3.8