Update python versions in README

[grahamu]

What's this PR do?

Update python versions in Environment Setup
Fix import sorting

Any background context you want to provide?

Update README for latest Python versions and revised testing instructions.
I'm not sure the whole CFLAGS= preamble is needed either, as I followed
the setup without that portion and tests worked fine.

Definition of Done (check if considered and/or addressed):

• Are all backwards incompatible changes documented in this PR?
• Have all new dependencies been documented in this PR?
• Has the appropriate documentation been updated (if applicable)?
• Have you written tests to prove this change works (if applicable)?

[paltman]

@grahamu this is failing linting because of the change to the python module. see https://circleci.com/gh/pinax/pinax-stripe/4072?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[codecov]

Codecov Report

Merging #463 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #463      +/-   ##
==========================================
+ Coverage    99.4%   99.52%   +0.12%     
==========================================
  Files          34       34              
  Lines        1682     1690       +8     
  Branches      144      145       +1     
==========================================
+ Hits         1672     1682      +10     
+ Misses          5        4       -1     
+ Partials        5        4       -1

Flag	Coverage Δ
#py27dj110	99.17% <ø> (-0.24%)	⬇️
#py27dj111	99.17% <ø> (-0.24%)	⬇️
#py27dj18	99.46% <ø> (+0.06%)	⬆️
#py34dj110	99.17% <ø> (-0.24%)	⬇️
#py34dj111	99.17% <ø> (-0.24%)	⬇️
#py34dj18	99.46% <ø> (+0.06%)	⬆️
#py34dj20	99.17% <ø> (-0.24%)	⬇️
#py35dj110	99.17% <ø> (-0.24%)	⬇️
#py35dj111	99.17% <ø> (-0.24%)	⬇️
#py35dj18	99.46% <ø> (+0.06%)	⬆️
#py35dj20	99.17% <ø> (-0.24%)	⬇️
#py36dj111	99.17% <ø> (-0.24%)	⬇️
#py36dj20	99.17% <ø> (-0.24%)	⬇️
#py36dj20psql	99.17% <ø> (-0.24%)	⬇️

Impacted Files	Coverage Δ
pinax/stripe/actions/charges.py	100% <0%> (ø)	⬆️
pinax/stripe/actions/events.py	100% <0%> (ø)	⬆️
pinax/stripe/views.py	100% <0%> (ø)	⬆️
pinax/stripe/models.py	100% <0%> (ø)	⬆️
pinax/stripe/middleware.py	100% <0%> (ø)	⬆️
pinax/stripe/forms.py	100% <0%> (+1.92%)	⬆️

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[paltman]

@grahamu i went ahead and took care of it. we should figure out how/why your local linting works differently than CI