Quote/Escape all URL placeholders

[llamasoft]

This MR fixes issue #528.

A new utility function, utils.format_url, has been added which can be used like a string's .format method but it applies urllib.parse.quote to all string-like placeholders. All api_path values that used .format have been updated to use the new function instead. For Python 2/3 compatibility it uses six library which is now included in the requirements file. All requirements files have been updated as tox fails when the requirements contain library version differences.

It should be noted that urllib.parse.quote doesn't escape forward slashes. I was extremely happy to find out that Vault handles all forward slashes correctly, even in cases where it might be confusing to a human. This means that no special handling is required for any placeholders regardless of the number of URL segments they should represent.

I also fixed a few minor formatting inconsistencies in api/secrets_engines/pki.pyand removed an unused placeholder indelete_root`.

Lastly, there is a potentially breaking change in api/auth_methods/aws.py function place_role_tags_in_blacklist where the misspelled argument "mount_pont" has been corrected.

[codecov-io]

Codecov Report

Merging #532 into develop will increase coverage by <.01%.
The diff coverage is 75.16%.

@@             Coverage Diff             @@
##           develop     #532      +/-   ##
===========================================
+ Coverage    82.84%   82.85%   +<.01%     
===========================================
  Files           54       54              
  Lines         2949     2968      +19     
===========================================
+ Hits          2443     2459      +16     
- Misses         506      509       +3

Impacted Files	Coverage Δ
hvac/api/secrets_engines/kv_v2.py	100% <100%> (ø)	⬆️
hvac/api/auth_methods/ldap.py	100% <100%> (ø)	⬆️
hvac/api/system_backend/audit.py	95.23% <100%> (+0.23%)	⬆️
hvac/utils.py	77.66% <100%> (+2.13%)	⬆️
hvac/api/auth_methods/okta.py	100% <100%> (ø)	⬆️
hvac/api/secrets_engines/pki.py	100% <100%> (ø)	⬆️
hvac/api/system_backend/lease.py	86.2% <100%> (+0.49%)	⬆️
hvac/api/system_backend/auth.py	87.17% <100%> (ø)	⬆️
hvac/api/secrets_engines/transit.py	93.23% <100%> (ø)	⬆️
hvac/api/secrets_engines/aws.py	98.07% <100%> (ø)	⬆️
... and 20 more

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[jeffwecan]

This looks like a great addition. Also good callout on the breaking change! We can make sure to call that out in the changelog notes on the off chance it would break someone's existing usage. 👍

[jeffwecan]

💯