build-wine: Bump Python to 3.8.7

Backported from Electrum 4.1.0.

contrib/build-wine/prepare-wine.sh

@@ -9,7 +9,7 @@ PYINSTALLER_REPO="https://github.com/SomberNight/pyinstaller.git"
 PYINSTALLER_COMMIT="6e455b2c1208465742484436009bfb1e1baf2e01"
 # ^ tag 4.0, plus a custom commit that fixes cross-compilation with MinGW
 
-PYTHON_VERSION=3.7.9
+PYTHON_VERSION=3.8.7
 
 ## These settings probably don't need change
 export WINEPREFIX=/opt/wine64
@@ -95,7 +95,13 @@ info "Building PyInstaller."
     echo "const char *electrum_tag = \"tagged by Electrum@$ELECTRUM_COMMIT_HASH\";" >> ./bootloader/src/pyi_main.c
     pushd bootloader
     # cross-compile to Windows using host python
-    python3 ./waf all CC=i686-w64-mingw32-gcc CFLAGS="-static -Wno-dangling-else -Wno-error=unused-value -Wno-error=implicit-function-declaration"
+    python3 ./waf all CC="${GCC_TRIPLET_HOST}-gcc" \
+                      CFLAGS="-static \
+                              -Wno-dangling-else \
+                              -Wno-error=unused-value \
+                              -Wno-error=implicit-function-declaration \
+                              -Wno-error=int-to-pointer-cast \
+                              -Wno-error=stringop-truncation"
     popd
     # sanity check bootloader is there:
     if [ "$WIN_ARCH" = "win32" ] ; then