Bump minimal boto3 version to 1.19.5

[hawflau]

RegionConfiguration.is_service_supported uses Session.get_partition_for_region, which requires boto3 v1.19.5 or above. Customer having older version of boto3 installed could get AttributeError: 'Session' object has no attribute 'get_partition_for_region'.

Issue #, if available:

Description of changes:

Description of how you validated changes:

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #2365 (7f5f448) into develop (e7a1496) will increase coverage by 0.71%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2365      +/-   ##
===========================================
+ Coverage    93.58%   94.29%   +0.71%     
===========================================
  Files           90       97       +7     
  Lines         6124     7116     +992     
  Branches      1260     1466     +206     
===========================================
+ Hits          5731     6710     +979     
- Misses         183      196      +13     
  Partials       210      210              

Impacted Files	Coverage Δ
samtranslator/__init__.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/dialup.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/feature_toggle.py	100.00% <100.00%> (+12.16%)	⬆️
samtranslator/intrinsics/actions.py	98.78% <100.00%> (+0.06%)	⬆️
samtranslator/intrinsics/resource_refs.py	95.83% <100.00%> (-0.17%)	⬇️
samtranslator/metrics/method_decorator.py	100.00% <100.00%> (ø)
samtranslator/metrics/metrics.py	100.00% <100.00%> (ø)
samtranslator/region_configuration.py	77.77% <0.00%> (-22.23%)	⬇️
samtranslator/validator/validator.py	91.80% <0.00%> (-3.85%)	⬇️
samtranslator/model/exceptions.py	97.67% <0.00%> (-2.33%)	⬇️
... and 43 more

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