Drop py37 support

[symonk]

Dropping support for python3.7. This encompasses:

• New python3.8+ syntax in places
• Dropping 3.7 support in infrastructure code
• Updating docs for any 3.7 references

closes #12399

[symonk]

not 100% the best approach to rolling this out, don't want to shock users using 3.7 without ample notice. But this is a start on the changes required

[codecov-commenter]

Codecov Report

Patch coverage: 93.10% and no project coverage change.

Comparison is base (09674ae) 57.01% compared to head (8844e9d) 57.02%.

❗ Current head 8844e9d differs from pull request most recent head a9ec7a9. Consider uploading reports for the commit a9ec7a9 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #12400   +/-   ##
=======================================
  Coverage   57.01%   57.02%           
=======================================
  Files          86       86           
  Lines        5335     5322   -13     
  Branches      198      192    -6     
=======================================
- Hits         3042     3035    -7     
  Misses       2095     2095           
+ Partials      198      192    -6     

Files Changed	Coverage Δ
py/selenium/common/exceptions.py	100.00% <ø> (ø)
py/selenium/types.py	75.00% <ø> (ø)
py/selenium/webdriver/chromium/options.py	77.01% <ø> (ø)
py/selenium/webdriver/chromium/webdriver.py	46.15% <ø> (ø)
py/selenium/webdriver/common/alert.py	53.84% <ø> (ø)
py/selenium/webdriver/common/by.py	100.00% <ø> (ø)
.../selenium/webdriver/common/desired_capabilities.py	100.00% <ø> (ø)
...lenium/webdriver/common/html5/application_cache.py	58.82% <ø> (ø)
py/selenium/webdriver/common/keys.py	100.00% <ø> (ø)
py/selenium/webdriver/common/options.py	34.31% <ø> (ø)
... and 21 more

... and 1 file with indirect coverage changes

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[AutomatedTester]

Do we have a sense of how many folk are on 3.7? I know its either EOL or about to be EOL

[titusfortner]

Eol was end of June.
My big thing is bazel won't let us test with it, so we don't have an easy way to ensure what we have works with 3.7 anyway.

[AutomatedTester]

The syntax changes in 3.7->3.8 is partly to blame. I'm ok with dropping it fully since it's past EOL

[AutomatedTester]

In general this looks good. There are a number of conflicts

[isaulv]

Most of the changes are style related but it's good to use the more modern type annotations. This should be good to go once conflicts have been addressed.

[symonk]

Looping back to this now, however i've some conflicts to resolve still, will squash as a single commit when ironed out