Retry page load errors

[jmbowman]

Changed PageLoadError and WrongPageError to inherit from WebDriverException so they'll trigger retries in wait_for_* method loops instead of failing the test outright; sometimes these get triggered while the browser is still in the middle of loading the next page.

Also ran make upgrade to update dependencies, which prompted a few other fixes:

• Make format strings Unicode to satisfy a new edx-pylint check
• Stop using some deprecated aspects of the selenium package API
• Consolidate dependency version restrictions in requirements/constraints.txt

[codecov-io]

Codecov Report

Merging #224 into master will increase coverage by 1.19%.
The diff coverage is 92.68%.

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   88.03%   89.22%   +1.19%     
==========================================
  Files          10       10              
  Lines         919      919              
  Branches      152      152              
==========================================
+ Hits          809      820      +11     
+ Misses         72       64       -8     
+ Partials       38       35       -3

Impacted Files	Coverage Δ
bok_choy/a11y/a11y_audit.py	95.74% <100%> (ø)	⬆️
bok_choy/a11y/axe_core_ruleset.py	95.28% <100%> (ø)	⬆️
bok_choy/page_object.py	97.28% <100%> (ø)	⬆️
bok_choy/web_app_test.py	85.52% <100%> (+9.21%)	⬆️
bok_choy/javascript.py	78.43% <100%> (ø)	⬆️
bok_choy/a11y/axs_ruleset.py	94.64% <100%> (ø)	⬆️
bok_choy/query.py	98.54% <100%> (ø)	⬆️
bok_choy/browser.py	71.69% <66.66%> (+1.88%)	⬆️

---

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 eed593a...eed686f. Read the comment docs.

[estute]

LGTM, thanks for catching this!