falcon: Remove or fix Python 3.4+ support

[adamantike]

The falcon instrumentation is the only one marked with support for Python versions other than >=3.6, allowing Python 3.4+.

However, since #693, f-strings were introduced to this repository, which should be breaking applications that want to use this instrumentation along with Python 3.4 (not tested).

Even more, considering that opentelemetry-sdk and opentelemetry-api are also marked with python_requires = >=3.6, it seems it's no longer possible to install the falcon installation on Python 3.4 anymore.

Example Dockerfile:

FROM python:3.4-slim
RUN pip install --no-cache-dir opentelemetry-instrumentation-falcon==0.25b2

Build logs:

$ docker build -t otel-falcon .                                                                                                                                                            
Sending build context to Docker daemon  15.16MB
Step 1/2 : FROM python:3.4-slim
3.4-slim: Pulling from library/python
f7e2b70d04ae: Pull complete
43e4f8f1b597: Pull complete
58d86f1ba87c: Pull complete
0a5fe6f498ca: Pull complete
36b630fe2a63: Pull complete
Digest: sha256:44b78f9c6c93df7d8bc0d32a85ef8fd10a074a101985bde6fd41d3b5aadf16f0
Status: Downloaded newer image for python:3.4-slim
 ---> 96a2eafeef70
Step 2/2 : RUN pip install --no-cache-dir opentelemetry-instrumentation-falcon==0.25b2
 ---> Running in 6174af410233
DEPRECATION: Python 3.4 support has been deprecated. pip 19.1 will be the last one supporting it. Please upgrade your Python as Python 3.4 won't be maintained after March 2019 (cf PEP 429).
Collecting opentelemetry-instrumentation-falcon==0.25b2
  Downloading https://files.pythonhosted.org/packages/8b/29/855d51027edab06e0b02760e23d9ac5552624ebfdda8e940b784f0b7636f/opentelemetry_instrumentation_falcon-0.25b2-py3-none-any.whl
Collecting opentelemetry-instrumentation-wsgi==0.25b2 (from opentelemetry-instrumentation-falcon==0.25b2)
  Could not find a version that satisfies the requirement opentelemetry-instrumentation-wsgi==0.25b2 (from opentelemetry-instrumentation-falcon==0.25b2) (from versions: 0.12b0)
No matching distribution found for opentelemetry-instrumentation-wsgi==0.25b2 (from opentelemetry-instrumentation-falcon==0.25b2)
You are using pip version 19.0.3, however version 19.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
The command '/bin/sh -c pip install --no-cache-dir opentelemetry-instrumentation-falcon==0.25b2' returned a non-zero code: 1

[owais]

The falcon instrumentation is the only one marked with support for Python versions other than >=3.6, allowing Python 3.4+.

This is a mistake. The project only supports Python 3.6+. We should update all pacakges to reflect that.