Line-item-manager

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* Require Python >=3.7 (3.6 EOL DEC 23 2021)

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* Update Google Ad Manager to v202105 (93)
* Remove Python 3.6 and add 3.10 support (91)
* Support for more line item types (sponsorship line item creation) (86)
* BUG FIX: Certain currency values are invalid (73)

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* Update Google Ad Manager to v202102 (63)
* Support Team ID (77)
* Support LineItem.videoMaxDuration required in v202102 (80)

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* BUG FIX: datetime.timezone objects did not include zone name (68)
* Support use of a custom line-item template (65)
* Update bumpversion (depracated) and pytest-runner (out-of-date) (62)

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* Support reportableType settings for CustomTargetingKey (55)
* Support for advertisers of different types (48)
* BUG FIX: Custom targeting hb_bidder contains all key-values (43)
* Upgrade jinja2 to 2.11.3 due to security vulnerabilities (51)
* Upgrade PyYAML due to security vulnerabilities (56)
* Remove requirements_dev.txt; not needed and introducing CVEs (58)

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* README includes steps to configure access to Google Ad Manager (39)
* README includes link to prebid documentation (40)