chore(sdk): add Python 3.7 EOL notice

[connor-mccarthy]

Description of your changes:
Adds Python 3.7 EOL warning when:

• The KFP SDK is imported into an environment running Python 3.7
• The dsl.component decorator is called without an explicit base_image argument

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this
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[google-oss-prow]

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[connor-mccarthy]

/assign @chensun

[chensun]

/lgtm

[google-oss-prow]

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This pull-request has been approved by: chensun

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