bpo-34022: Stop forcing of hash-based invalidation with SOURCE_DATE_EPOCH (GH-9607)

Unconditional forcing of ``CHECKED_HASH`` invalidation was introduced in
3.7.0 in bpo-29708.  The change is bad, as it unconditionally overrides
*invalidation_mode*, even if it was passed as an explicit argument to
``py_compile.compile()`` or ``compileall``.  An environment variable
should *never* override an explicit argument to a library function.
That change leads to multiple test failures if the ``SOURCE_DATE_EPOCH``
environment variable is set.

This changes ``py_compile.compile()`` to only look at
``SOURCE_DATE_EPOCH`` if no explicit *invalidation_mode* was specified.
I also made various relevant tests run with explicit control over the
value of ``SOURCE_DATE_EPOCH``.

While looking at this, I noticed that ``zipimport`` does not work
with hash-based .pycs _at all_, though I left the fixes for
subsequent commits.

Lib/py_compile.py

@@ -69,8 +69,15 @@ class PycInvalidationMode(enum.Enum):
     UNCHECKED_HASH = 3
 
 
+def _get_default_invalidation_mode():
+    if os.environ.get('SOURCE_DATE_EPOCH'):
+        return PycInvalidationMode.CHECKED_HASH
+    else:
+        return PycInvalidationMode.TIMESTAMP
+
+
 def compile(file, cfile=None, dfile=None, doraise=False, optimize=-1,
-            invalidation_mode=PycInvalidationMode.TIMESTAMP):
+            invalidation_mode=None):
     """Byte-compile one Python source file to Python bytecode.
 
     :param file: The source file name.
@@ -112,8 +119,8 @@ def compile(file, cfile=None, dfile=None, doraise=False, optimize=-1,
     the resulting file would be regular and thus not the same type of file as
     it was previously.
     """
-    if os.environ.get('SOURCE_DATE_EPOCH'):
-        invalidation_mode = PycInvalidationMode.CHECKED_HASH
+    if invalidation_mode is None:
+        invalidation_mode = _get_default_invalidation_mode()
     if cfile is None:
         if optimize >= 0:
             optimization = optimize if optimize >= 1 else ''