Using `|=` here makes `trace_flags` into an int, rather than a
`TraceFlags` object:

```
>>> import opentelemetry.trace as trace
>>> f = trace.TraceFlags()
>>> type(f)
<class 'opentelemetry.trace.span.TraceFlags'>
>>> f |= trace.TraceFlags.SAMPLED
>>> type(f)
<class 'int'>
```

This fix will preserve the type.

You may ask, but where's the sample code to demostrate the bug?  I'm in
the middle of trying to debug a number of odd issues with the gRPC
client interceptor, so it's a bit convoluted, but it looks like this:

```
Traceback (most recent call last):
  File "/home/michael/work/myapp/.venv/lib/python3.8/site-packages/grpc/_server.py", line 435, in _call_behavior
    response_or_iterator = behavior(argument, context)
  File "/home/michael/xfer/opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/_server.py", line 252, in telemetry_interceptor
    with self._start_span(
  File "/home/michael/xfer/opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/_server.py", line 232, in _start_span
    return self._tracer.start_as_current_span(
  File "/home/michael/xfer/opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py", line 756, in start_as_current_span
    span = self.start_span(
  File "/home/michael/xfer/opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py", line 807, in start_span
    sampling_result = self.sampler.should_sample(
  File "/home/michael/xfer/opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/trace/sampling.py", line 273, in should_sample
    if parent_span_context.trace_flags.sampled:
AttributeError: 'int' object has no attribute 'sampled'
```