test(python): use constraints files to check dependency lower bounds

Use a constraints file when installing dependencies for system and unit tests nox sessions.

https://pip.pypa.io/en/stable/user_guide/#constraints-files
> Constraints files are requirements files that **only control which version of a requirement is installed, not whether it is installed or not**. Their syntax and contents is nearly identical to Requirements Files. There is one key difference: Including a package in a constraints file does not trigger installation of the package.

```
testing
├── constraints-3.10.txt
├── constraints-3.11.txt
├── constraints-3.6.txt
├── constraints-3.7.txt
├── constraints-3.8.txt
└── constraints-3.9.txt
```

Going forward, one constraints file (currently 3.6) will be populated with every library requirement and extra listed in the `setup.py`. The constraints file will pin each requirement to the lower bound. This ensures that library maintainers will see test failures if they forget to update a lower bound on a dependency.

See googleapis/python-bigquery#263 for an example

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Tue Mar 23 10:52:02 2021 -0600
Source-Repo: googleapis/synthtool
Source-Sha: 86ed43d4f56e6404d068e62e497029018879c771
Source-Link: googleapis/synthtool@86ed43d

Author: yoshi-automation

noxfile.py

@@ -18,6 +18,7 @@
 
 from __future__ import absolute_import
 import os
+import pathlib
 import shutil
 
 import nox
@@ -30,6 +31,8 @@
 SYSTEM_TEST_PYTHON_VERSIONS = ["2.7", "3.8"]
 UNIT_TEST_PYTHON_VERSIONS = ["2.7", "3.6", "3.7", "3.8", "3.9"]
 
+CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
+
 # 'docfx' is excluded since it only needs to run in 'docs-presubmit'
 nox.options.sessions = [
     "unit",
@@ -84,11 +87,13 @@ def lint_setup_py(session):
 
 def default(session):
     # Install all test dependencies, then install this package in-place.
-    session.install(
-        "mock", "pytest", "pytest-cov",
+
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
     )
+    session.install("mock", "pytest", "pytest-cov", "-c", constraints_path)
 
-    session.install("-e", ".")
+    session.install("-e", ".", "-c", constraints_path)
 
     # Run py.test against the unit tests.
     session.run(
@@ -115,6 +120,9 @@ def unit(session):
 @nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
 def system(session):
     """Run the system test suite."""
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
     system_test_path = os.path.join("tests", "system.py")
     system_test_folder_path = os.path.join("tests", "system")
 
@@ -139,10 +147,8 @@ def system(session):
 
     # Install all test dependencies, then install this package into the
     # virtualenv's dist-packages.
-    session.install(
-        "mock", "pytest", "google-cloud-testutils",
-    )
-    session.install("-e", ".")
+    session.install("mock", "pytest", "google-cloud-testutils", "-c", constraints_path)
+    session.install("-e", ".", "-c", constraints_path)
 
     # Run py.test against the system tests.
     if system_test_exists:

synth.metadata

@@ -11,7 +11,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "f5c5904fb0c6aa3b3730eadf4e5a4485afc65726"
+        "sha": "86ed43d4f56e6404d068e62e497029018879c771"
       }
     }
   ],