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
 

doc/build-unix.md

@@ -65,7 +65,7 @@ Dependency Build Instructions: Ubuntu & Debian
 ----------------------------------------------
 Build requirements:
 
-    sudo apt-get install build-essential libtool autotools-dev automake pkg-config libssl-dev libevent-dev bsdmainutils
+    sudo apt-get install build-essential libtool autotools-dev automake pkg-config libssl-dev libevent-dev bsdmainutils python3
 
 Options when installing required Boost library files:
 
@@ -131,7 +131,7 @@ Dependency Build Instructions: Fedora
 -------------------------------------
 Build requirements:
 
-    sudo dnf install gcc-c++ libtool make autoconf automake openssl-devel libevent-devel boost-devel libdb4-devel libdb4-cxx-devel
+    sudo dnf install gcc-c++ libtool make autoconf automake openssl-devel libevent-devel boost-devel libdb4-devel libdb4-cxx-devel python3
 
 Optional: