lib: set version.TESTING based on if the unittest module is loaded

This is cleaner than needing to set an environment variable before
running unit tests.

Implements #28 (Check if unittest is loaded to determine if we are testing)
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

Makefile

@@ -37,4 +37,4 @@ pkgbuild:
 .PHONY: tests
 tests:
 	python tools/generate_tracks.py
-	EMMENTAL_TESTING=1 python -m unittest discover -v
+	python -m unittest discover -v

lib/version.py

@@ -1,5 +1,6 @@
 # Copyright 2021 (c) Anna Schumaker.
 import os
+import sys
 
 MAJOR = 2
 MINOR = 9
@@ -10,7 +11,7 @@ if os.path.exists(".debug"):
         if f.read().strip() == "emmental":
             DEBUG = True
 
-TESTING = os.environ.get("EMMENTAL_TESTING") != None
+TESTING = "unittest" in sys.modules
 
 
 def string():