Add Tidelift to documentation and urllib3 branding

[sethmlarson]

I think it's totally fine to get rid of the major header in our README. Sphinx documentation isn't modified with a banner.

[codecov-io]

Codecov Report

Merging #1735 into master will decrease coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1735      +/-   ##
==========================================
- Coverage    99.7%   99.04%   -0.66%     
==========================================
  Files          22       22              
  Lines        2003     1988      -15     
==========================================
- Hits         1997     1969      -28     
- Misses          6       19      +13

Impacted Files	Coverage Δ
src/urllib3/contrib/_appengine_environ.py	70% <0%> (-10%)	⬇️
src/urllib3/util/wait.py	93.75% <0%> (-6.25%)	⬇️
src/urllib3/connection.py	93.75% <0%> (-4.2%)	⬇️
src/urllib3/response.py	99.44% <0%> (-0.56%)	⬇️
src/urllib3/util/url.py	99.47% <0%> (-0.02%)	⬇️
src/urllib3/util/timeout.py	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6996da3...fbd04ec. Read the comment docs.