To avoid generating bogus PRs

Co-authored-by: Jeffrey Rennie <rennie@google.com>

Source-Author: Carlos de la Guardia <cguardia@yahoo.com>
Source-Date: Tue May 19 11:57:19 2020 -0500
Source-Repo: googleapis/synthtool
Source-Sha: d2364eb80b840a36136c8ce12f1c6efabcc9600e
Source-Link: googleapis/synthtool@d2364eb

Add a lower bound checker to be invoked on the command line by client libraries. This accomplishes two things:
- Makes it easier to mass update/create constraints files.
- Adds another layer of checks to the lower bounds in setup.py / constraints files.

See googleapis/synthtool#869 for additional context on the lower bounds work.

`lower-bound-checker` is intentionally restrictive on the types of pins it permits in `setup.py` and `constraints.txt`. The tool doesn't try to `pip install` the package. It is assumed that other checks (e.g., setup.py lint, unit tests) will pick up on things like misspelled package names or a version range that cannot be fulfilled.

```
busunkim@busunkim:~/github/python-test-utils$ lower-bound-checker check --help
Usage: lower-bound-checker check [OPTIONS]

  Check that the constraints-file pins to the lower bound specified in
  package-name's setup.py for each requirement.

  Requirements:

  1. The setup.py pins every requirement in one of the following formats:

      * foo==1.2.0

      * foo>=1.2.0

      * foo>=1.2.0, <2.0.0dev

      * foo<2.0.0dev, >=1.2.0

  2. The constraints file pins every requirement to a single version:

      * foo==1.2.0

  3. package-name is already be installed in the environment.

Options:
  --package-name TEXT      Name of the package.  [required]
  --constraints-file TEXT  Path to constraints file.  [required]
  --help                   Show this message and exit.
```


**Example Usage:**

(for reviewers)
- Install this repo from source: `pip install /path/to/repo`
- Install another package from source: `pip install /path/to/google-cloud-foo`

1. Check an existing package and constraints file:
```
lower-bound-checker check --package-name google-cloud-foo
--constraints-file testing/constraints-3.6.txt
```

2. Create/update a constraints file:
```
lower-bound-checker update --package-name google-cloud-foo
--constraints-file testing/constraints-3.6.txt
```

🤖 I have created a release \*beep\* \*boop\* 
---
## [0.2.0](https://www.github.com/googleapis/python-test-utils/compare/v0.1.0...v0.2.0) (2021-02-22)


### Features

* add lower bound checker ([#8](https://www.github.com/googleapis/python-test-utils/issues/8)) ([5ebac9f](https://www.github.com/googleapis/python-test-utils/commit/5ebac9fb0ad005f8ea947c14dfca6de3c0d2cac9))
---


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