Add the CI to verify Python 3.7 and the fastavro dependency

[BewareMyPower]

Motivation

There is no workflow to verify the fastavro dependency works well for all supported Python versions, so the [avro] component was broken and not found before. See #67

Modifications

• In all workflows, install the wheel with the [avro] component and verify pulsar.schema can be imported successfully.
• Add workflows for the lowest Python version (3.7) we supported.

[BewareMyPower]

It seems that there is something wrong with the Python 3.7 build on macOS

WARNING: pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available.
Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLError("Can't connect to HTTPS URL because the SSL module is not available.")) - skipping
+ /Users/runner/work/pulsar-client-python/pulsar-client-python/.pulsar-mac-build/deps/install/bin/python3 -c 'import pulsar; c = pulsar.Client("pulsar://localhost:6650"); c.close()'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/runner/work/pulsar-client-python/pulsar-client-python/.pulsar-mac-build/deps/install/lib/python3.7/site-packages/pulsar/__init__.py", line 459, in __init__
    import certifi
ModuleNotFoundError: No module named 'certifi'

[BewareMyPower]

I think we need to build Python with the SSL support for macOS build. I will fix it soon.

[tisonkun]

It seems the error remains after the last patch.

[BewareMyPower]

It seems the error remains after the last patch.

Yes. I need to verify it in my local env first. Recently I don't have much time for this issue.