Maintenance Patch 2 for 8.0U1

Bindings:

• Added bindings and type hints for Virtual Storage Lifecycle Management for vSphere 8.0U1 - VSLM namespace

Changes:

• Fixed: #1021 - Switch to static imports for type info modules
• Fixed: #1022 - Support proxy authentication
• Add custom HTTP headers support to connect.SmartStubAdapter()
• Missing filters are no longer treated as task failures
• Various small fixes for docstrings and linter checks

Type Hints Fixes:

• Fixed: #1026 - Use the "from Y import X as X" format to re-export submodules
• Fixed: #1030 - Use a fully qualified name when the type is from another namespace/package

Tests:

• vcrpy dependency is updated to the latest version with Python 2 support
• testtools dependency is removed
• Travis CI is no longer used
• Various test updates and fixes. All tests are enabled.