Add DNS cache

[Evanito]

Closes #597 Tested it, and it seems to work pretty well. Only gets the DNS once, then instantly reuses it until it checks again at the start of the next cycle.

Also adds venv created by my IDE to the .gitignore

Description

Closes #597

Reduces DNS requests from several hundred a minute to only a few in the same time.

TESTING STAGE

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I fully understand Github Flow.
• [x] My code adheres to the code style of this project.
• [N/A] I have updated the documentation in /docs if I have changed the config, arguments, logic in how the bot works, or anything that understandably needs a documentation change.
• [N/A] I have updated the config file accordingly if my change requires a new configuration setting or changes an existing one.
• [x] I have tested the bot with no issues for 24 continuous hours. If issues were experienced, they have been patched and tested again.

[Evanito]

Vertical axis is DNS requests per 10 minutes. Look at the very right of the chart, where I have enabled this PR. The bot is in orange.

That is one heck of an improvement, I'd say.

[Evanito]

Occasionally getting API Error 521: The web server reported a bad gateway or gateway timeout error. May not be related to PR since there was also an issue accessing Polo in browser.

Maybe have it flush DNS cache on any 5XX error to be safe?

Edit: Also getting the error on master. Edit2: Error resolved self over time, does not look like this PR's fault.

[Evanito]

Bot is working happily with only a minute percentage of the DNS requests