issue #291: don't attempt mitogen import until sys.path modified.

Given an extracted download of mitogen-2.2.tar.gz, with strategy_plugins
pointing into it, if an old version of the package was pip-installed,
then the old pip-installed package would be imported and override
whatever came from the tarball.

Instead, modify sys.path before attempting any import. This still isn't
perfect, but it's better.

ansible_mitogen/plugins/strategy/mitogen.py

@@ -44,12 +44,12 @@ import sys
 # debuggers and isinstance() work predictably.
 #
 
-try:
-    import ansible_mitogen
-except ImportError:
-    base_dir = os.path.dirname(__file__)
-    sys.path.insert(0, os.path.abspath(os.path.join(base_dir, '../../..')))
-    del base_dir
+BASE_DIR = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), '../../..')
+)
+
+if BASE_DIR not in sys.path:
+    sys.path.insert(0, BASE_DIR)
 
 import ansible_mitogen.strategy
 import ansible.plugins.strategy.linear

ansible_mitogen/plugins/strategy/mitogen_free.py

@@ -44,12 +44,12 @@ import sys
 # debuggers and isinstance() work predictably.
 #
 
-try:
-    import ansible_mitogen
-except ImportError:
-    base_dir = os.path.dirname(__file__)
-    sys.path.insert(0, os.path.abspath(os.path.join(base_dir, '../../..')))
-    del base_dir
+BASE_DIR = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), '../../..')
+)
+
+if BASE_DIR not in sys.path:
+    sys.path.insert(0, BASE_DIR)
 
 import ansible_mitogen.loaders
 import ansible_mitogen.strategy

ansible_mitogen/plugins/strategy/mitogen_linear.py

@@ -44,12 +44,12 @@ import sys
 # debuggers and isinstance() work predictably.
 #
 
-try:
-    import ansible_mitogen
-except ImportError:
-    base_dir = os.path.dirname(__file__)
-    sys.path.insert(0, os.path.abspath(os.path.join(base_dir, '../../..')))
-    del base_dir
+BASE_DIR = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), '../../..')
+)
+
+if BASE_DIR not in sys.path:
+    sys.path.insert(0, BASE_DIR)
 
 import ansible_mitogen.loaders
 import ansible_mitogen.strategy