Release MyGrad 2.1.0

See detailed changelog here.

What's Changed

• update docs dependencies, fix deprecated css, fix doc typo by @rsokl in #376
• Use faster dict-construction syntax by @rsokl in #377
• Update default behavior of derivative of linalg.norm and abs at x=0 by @rsokl in #379
• add nightly tests against pre-releases by @rsokl in #380
• Adds mygrad.save and mygrad.load by @rsokl in #383
• accommodates hypothesis #3065 by @rsokl in #385
• remove bad example from docs; improve type annotations by @rsokl in #386
• Implement atleast_kd by @rsokl in #387
• Update test_duplicating_graph.py by @rsokl in #388
• test should not pass None to array(..., copy=<>) by @rsokl in #390
• fix broadcasting hack by @rsokl in #391
• NEP 29: min numpy is now 1.19 by @rsokl in #394
• add Tensor.index method by @rsokl in #395
• Implement clip(..., out=) by @rsokl in #396
• Docs for 2.1.0 by @rsokl in #397

Full Changelog: v2.0.2...v2.1.0