Add python3 to list of dependencies on some platforms

python3 is required for running the unit tests on macOS, Ubuntu and Fedora.

Without python3 installed, 'make check' fails because /test/util/bitcoin-util-test.py fails to find python3.

doc/build-osx.md

@@ -16,7 +16,7 @@ Then install [Homebrew](https://brew.sh).
 Dependencies
 ----------------------
 
-    brew install automake berkeley-db4 libtool boost --c++11 miniupnpc openssl pkg-config protobuf qt libevent
+    brew install automake berkeley-db4 libtool boost --c++11 miniupnpc openssl pkg-config protobuf python3 qt libevent
 
 If you want to build the disk image with `make deploy` (.dmg / optional), you need RSVG