Devpi-findlinks

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- Drop support for Python < 3.7. Support for Python versions will be dropped
with their respective end of lives.

- Fix plugin registration with pluggy >= 1.0.0.

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- fix use of eggfragment for devpi-server >= 5.0.0.

- dropped testing with Python 2.6 and Python 3.4

- added testing with Python 3.6, 3.7 and 3.8

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- fixes for devpi-server 3.0.0, older versions aren't supported anymore

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- fixes for devpi-server 2.2.0, older versions aren't supported anymore

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- initial release