[Dependency] Drop Python 3.6 support, switch to Python>=3.7 and fix azure launching issue

[Michaelvll]

Our previous patch for the azure-identity is outdated and causing trouble launching azure cluster. Since the latest azure-identity contains the process_timeout argument, we can directly use that to solve the problem we mentioned in

skypilot/sky/skylet/ray_patches/__init__.py

Lines 88 to 91 in bbdd5b1

	# Fix the Azure get-access-token (used by ray azure node_provider) timeout issue,
	# by increasing the timeout.
	# Tracked in https://github.com/Azure/azure-cli/issues/20404#issuecomment-1249575110
	# Only patch it if azure cli is installed.

Also, closes #609

Based on #1954

Tested (run the relevant ones):

• Any manual or new tests for this PR (please specify below)
  • sky launch -c test-azure --cloud azure --cpus 4+
  • sky launch -c test-azure-2 --cloud azure --cpus 4+ --num-nodes 2
• All smoke tests: pytest tests/test_smoke.py
• All smoke tests: pytest tests/test_smoke.py --aws

[concretevitamin]

Thanks @Michaelvll. Tested launching on a previously stopped GCP cluster.

Before merging, can we update the ">= 3.6" note in the following places?

» ag '>= 3.6'
sky/design_docs/onprem-design.md
13:- Python >= 3.6 for all users.

docs/source/getting-started/installation.rst
8:  $ # SkyPilot requires python >= 3.6. For Apple Silicon, use >= 3.8.

docs/source/reference/local/setup.rst
25:   $ # SkyPilot requires python >= 3.6.

CONTRIBUTING.md
19:# SkyPilot requires python >= 3.6.

Some of these duplicate notes probably can be dropped.

[concretevitamin]

Nit: maybe change title to be something like "Drop Python 3.6 requirement and allow >= 3.7 ...", just to ease future release notes writing.