S: can't use u prefixes, as it is syntax error there

* PkgConfigNotFound             - not installed
* PkgConfigError                - base pkg-config errors
    * PkgConfigModuleNotFound   - pc file for module was not found
    * PkgConfigModuleVersionNotFound - requested version was not found

Boilerplate now looks
```
from cffi.error import PkgConfigNotFound, PkgConfigError
...
try:
    # try pkg-config way
    ffibuilder.set_source(..., pkgconfig=["libczmq >= 4.0.0"])
except PkgConfigNotFound as e:
    # if pkg-config is not installed, try backup
    ffibuilder.set_source(..., libraries=["czmq", "zmq", "uuid", "pgm"])
except PkgConfigError as e:
    # here we catch both PkgConfigModuleNotFound and PkgConfigModuleVersionNotFound
    # and raise it again - simply to show they are raised
    raise e from None
```

Now the with non pkg-config backup would be
```
module_name = "_czmq"
source = "#include <czmq.h>"
try:
    print(f"### pkg-config path")
    ffibuilder.set_source(
        module_name,
        source,
        pkgconfig=["libczmq"]
    )
except Exception as e:
    print(f"Exception e: {e}")
    ffibuilder.set_source(
        module_name,
        source,
        libraries=["czmq"]
    )
```

Fix encoding errors

Given testing Python program
```
from cffi import FFI
ffibuilder = FFI()

ffibuilder.cdef(
    "char* zsys_hostname();"
)

ffibuilder.set_source(
    "_czmq",
    "#include <czmq.h>",
    pkgconfig=["libczmq"]
)


if __name__ == "__main__":
    ffibuilder.compile(verbose=True)
```

We can run ffibuilder from source dir of czmq

```
PKG_CONFIG_PATH=`pwd`/src python3 t.py
generating ./_czmq.c
...
gcc -pthread -shared -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none ./_czmq.o -L/usr/local/lib64 -L/usr/lib64 -lczmq -lzmq -lpython3.6m -o ./_czmq.cpython-36m-x86_64-linux-gnu.so
```

```
python3 t.py
generating ./_czmq.c
...
gcc -pthread -shared -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none ./_czmq.o -L/usr/lib64 -lczmq -lpython3.6m -o ./_czmq.cpython-36m-x86_64-linux-gnu.so
```

Note that in the first case `/usr/local` has been added to the compiler path as
provided by local pkg-config file.

Making `pkgconfig.call` function accessible, tests can monkey patch it and
provide mock. This improves testing, however raised a need to give
functions better names than `pkgconfig.pkgconfig_kwargs` or `pkgconfig.pc`.

use bytes instead of strings