Update to PyTorch 1.11.0

[fritzo]

This is long overdue: before this PR Pyro's dev branch was still on torch==1.9.0.

• Bump Python version to 3.7, drops support for 3.6 (which reached end of life last December)
• Bump PyTorch to 1.11.0, torchvision to 0.12.0
  • Convert torch.triangular_solve() -> torch.linalg.solve_triangular()
  • Ignore irrelevant __floordiv__ warnings
  • Fix catching of singular matrices in hmc
  • Resolves [CI] torch.meshgrid having new default indexing in 1.10.0 (CI server currently installs 1.9.0) #3032 (meshgrid workaround)
  • Resolves updated classifiers #3035
  • Remove an XPASSing jit test
  • Xfailed some funsor tests, these are collected in Some pyro.contrib.funsor tests are failing in torch==1.11.0 #3046
  • Work around a pandas>=1.3 bug in the baseball example
• Update to latest black

I've tried to keep each git commit narrowly focused on a single change.

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[fehiepsi]

Thanks for fixing the issues, @fritzo! Looks clean to me.

[fritzo]

Thanks @fehiepsi!