Python-Bindings: Subsequent local build and install fails

if the MythTVs Python-Bindings of the same version are already installed.

Solution:
Add the switch `--ignore-installed` to the `pip install` sequence,
which insatll the generated python-wheel file unconditionally.

Thanks to Mark S. for reporting this issue and providing hints to solve it.

Fixes #843

mythtv/bindings/python/Makefile

@@ -32,7 +32,7 @@ python_build:
 	$(PYTHON) -m pip wheel $(PIP_OPTIONS) --wheel-dir ./$(WHEEL_DIR) .
 
 install:
-	$(PYTHON) -m pip install $(ROOT_FLAGS) $(PREFIX_FLAGS) $(PIP_OPTIONS) --find-links ./$(WHEEL_DIR) MythTV
+	$(PYTHON) -m pip install $(ROOT_FLAGS) $(PREFIX_FLAGS) $(PIP_OPTIONS) --ignore-installed --find-links ./$(WHEEL_DIR) MythTV
 
 uninstall:
 	$(warning python pip uninstall is not supported for python bindings)