Allow bypassing requirements installation.

This shall be considered an expert feature since you need to be aware of
what's going on.
This is useful when writing tests and running them over and over again
where on the first run the requirements get properly installed but on
subsequent runs, if no requirements are changed, the install procedure
does not need to run, thus, saving valuable seconds which can quick turn
into several minutes when repeating test runs.

noxfile.py

@@ -42,6 +42,7 @@ CI_RUN = (
     or os.environ.get("DRONE") is not None
 )
 PIP_INSTALL_SILENT = CI_RUN is False
+SKIP_REQUIREMENTS_INSTALL = "SKIP_REQUIREMENTS_INSTALL" in os.environ
 
 # Global Path Definitions
 REPO_ROOT = os.path.abspath(os.path.dirname(__file__))
@@ -291,6 +292,11 @@ def _get_distro_pip_constraints(session, transport):
 
 
 def _install_requirements(session, transport, *extra_requirements):
+    if SKIP_REQUIREMENTS_INSTALL:
+        session.log(
+            "Skipping Python Requirements because SKIP_REQUIREMENTS_INSTALL was found in the environ"
+        )
+        return
     # Install requirements
     distro_constraints = _get_distro_pip_constraints(session, transport)
 
@@ -354,7 +360,10 @@ def _install_requirements(session, transport, *extra_requirements):
 
 
 def _run_with_coverage(session, *test_cmd):
-    session.install("--progress-bar=off", "coverage==5.0.1", silent=PIP_INSTALL_SILENT)
+    if SKIP_REQUIREMENTS_INSTALL is False:
+        session.install(
+            "--progress-bar=off", "coverage==5.0.1", silent=PIP_INSTALL_SILENT
+        )
     session.run("coverage", "erase")
     python_path_env_var = os.environ.get("PYTHONPATH") or None
     if python_path_env_var is None: