avoid tracebacks / ignore errors from too low version SSL clients

[sanderjo]

with help by @Safihre

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

#292

❓ What is the current behavior? (You can also link to an open issue here)

A TLSv1 connection like
echo | ~/git/testssl.sh/bin/openssl.Linux.x86_64 s_client -connect 127.0.0.1:8080 -tls1 2>&1
gives

2020-11-14 12:07:56,135::INFO::[notifier:122] Sending notification: Error - [14/Nov/2020:12:07:56] ENGINE Error in HTTPServer.tick
Traceback (most recent call last):
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/server.py", line 1788, in serve
    self.tick()
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/server.py", line 2023, in tick
    conn = self.connections.get_conn(self.socket)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/connections.py", line 188, in get_conn
    return self._from_server_socket(server_socket)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/connections.py", line 207, in _from_server_socket
    s, ssl_env = self.server.ssl_adapter.wrap(s)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/ssl/builtin.py", line 277, in wrap
    s = self.context.wrap_socket(
  File "/usr/lib/python3.8/ssl.py", line 500, in wrap_socket
    return self.sslsocket_class._create(
  File "/usr/lib/python3.8/ssl.py", line 1040, in _create
    self.do_handshake()
  File "/usr/lib/python3.8/ssl.py", line 1309, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: UNSUPPORTED_PROTOCOL] unsupported protocol (_ssl.c:1123)

and a SSLv3 connection like
echo | ~/git/testssl.sh/bin/openssl.Linux.x86_64 s_client -connect 127.0.0.1:8080 -ssl3 2>&1
gives

2020-11-14 12:08:23,233::INFO::[notifier:122] Sending notification: Error - [14/Nov/2020:12:08:23] ENGINE Error in HTTPServer.tick
Traceback (most recent call last):
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/server.py", line 1788, in serve
    self.tick()
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/server.py", line 2023, in tick
    conn = self.connections.get_conn(self.socket)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/connections.py", line 188, in get_conn
    return self._from_server_socket(server_socket)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/connections.py", line 207, in _from_server_socket
    s, ssl_env = self.server.ssl_adapter.wrap(s)
  File "/home/sander/.local/lib/python3.8/site-packages/cheroot/ssl/builtin.py", line 277, in wrap
    s = self.context.wrap_socket(
  File "/usr/lib/python3.8/ssl.py", line 500, in wrap_socket
    return self.sslsocket_class._create(
  File "/usr/lib/python3.8/ssl.py", line 1040, in _create
    self.do_handshake()
  File "/usr/lib/python3.8/ssl.py", line 1309, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: VERSION_TOO_LOW] version too low (_ssl.c:1123)

❓ What is the new behavior (if this is a feature change)?

No more traceback

📋 Other information:

📋 Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

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