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Cleanup and update documentation.

• update requirements, including the ones used for testing
• support python 2.7, 3.4, 3.5, 3.6, 3.7 and 3.8
• test and add support for ubuntu 18.04
• format everything with black

• fix typo for max_open_files argument on mount

• add max_open_files mount option which allow to specify a files open hard and soft limit #259

• show a helpful message when trying to clone an empty repository #239
• log exception only on failed push #240
• add sentry tags #244
• upgrade pygit2 to 0.24.1

• improve commit message thanks to @tusharmakkar08

• fix pending writers bug #223
• cleanup code and error messages (thanks to @promulo and @ChristianAlexander)

• add credentials to push and fetch operations #226 (hotfix)

• delete directories on force checkout #221
• improve logging #222
• retry mechanism for push #201
• port to python3 thanks to @justuswilhelm #199
• improve test running speed thanks to @rciorba
• improve development using vagrant