Try to use dotted import of fallback in depends

Author: g1itch

src/depends.py

@@ -191,7 +191,7 @@ def try_import(module, log_extra=False):
 def check_ripemd160():
     """Check availability of the RIPEMD160 hash function"""
     try:
-        from fallback import RIPEMD160Hash
+        from .fallback import RIPEMD160Hash
     except ImportError:
         return False
     return RIPEMD160Hash is not None
@@ -385,7 +385,7 @@ def check_pyqt():
     """
     # pylint: disable=no-member
     try:
-        from fallback import PyQt5
+        from .fallback import PyQt5
     except ImportError:
         logger.error(
             'PyBitmessage requires PyQt5 or qtpy, PyQt 4.8 or later'