Update dependency environs to v15.2.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
environs (changelog)	==15.0.1 → ==15.2.0

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Release Notes

sloria/environs (environs)

v15.2.0

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Features:

• Export exception classes in environs.__all__ (#​488).
  Thanks paduszyk for the suggestion and the PR.

Bug fixes:

• Return None for invalid values when eager=False, matching the documented
  T | None return type and the missing-variable case (#​486).
  Thanks VANDRANKI for reporting and gyanu2507
  for the PR.

v15.1.0

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Features:

• Add a specific exception, EnvNotSetError, for missing environment variables (#​480).
  Thanks paduszyk for the suggestion and w3lld1 for the PR.

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