[tfenv use] latest-allowed should work on exact constraints

[Sickafant]

There should be parity in behaviour for exact version values when it comes to the version used and picked up in the terraform block when using min-required and latest-allowed.

Problem:

➜  dummy cat versions.tf
terraform {
  required_version = "1.3.9"
}

➜  dummy tfenv use min-required
Switching default version to v1.3.9
Default version (when not overridden by .terraform-version or TFENV_TERRAFORM_VERSION) is now: 1.3.9

➜  dummy tfenv use latest-allowed
Unsupported version spec: '1.3.9', only >, >=, <=, and ~> are supported.
Failure to resolve version from latest-allowed

Expected:

➜  dummy cat versions.tf
terraform {
  required_version = "1.3.9"
}

➜  dummy tfenv use min-required
Switching default version to v1.3.9
Default version (when not overridden by .terraform-version or TFENV_TERRAFORM_VERSION) is now: 1.3.9

➜  dummy tfenv use latest-allowed
Switching default version to v1.3.9
Default version (when not overridden by .terraform-version or TFENV_TERRAFORM_VERSION) is now: 1.3.9

[jmnavarrol]

This seems a duplicate (or maybe a subset) of #367.